First off, Welcome to Sunline Owners club!!! Congrats on your Sunny. I had some time tonight so I typed to you on your questions as these have been coming up lately on how to extend the life of your camper. This may be a lot more then you thought, read it over as time allows.
Now to your seams. The sealants of the camper are a weak spot. I do not know exactly where on the camper you are referring too so I will expand a little for you to confirm. If you can post pics, it will greatly help confirm we can see what you are up against and help better. Ask away any questions and or new clarifications.
Your camper has a rubber roof that unless special ordered from the factory is not a direct walk on roof. Means you need to take precautions and some extra steps before walking up the to do service work.
The roof uses "butyl" sealing tape against the rubber under aluminum flanges that hold the roof on. You cannot see this butyl tape but it is there. What you can see is the self leveling caulking which creates a seal to the outside world on the roof. This caulk is called Dicor Self Leveling caulk. It comes in different colors, white is what it normally used . See here https://dicorproducts.com/product/ep.../#installation
You can buy it on line at many places that sells RV parts. This post can help on how to spot tiny and large cracks in the caulking. http://www.sunlineclub.com/forums/f7...1-a-11508.html
For the roof to stay in great condition and not deteriorate it needs to be cleaned regularly and the roof caulking checked and touched up routinely as well. It is recommended 4 times a year on both if your camper lives outside all the time with special highlights on the sealant check going into winter and coming out of winter. The 2 checks during the summer time can be as time fits.
Aluminum molding, entry door, window flanges, cargo hole doors, furnaces, hot water heater, and other penetrations into the siding of the camper sealants.
On all the penetrations into the siding of the camper, a sealant called "putty tape" is used on the flanges of the object you are trying to seal. This putty tape works well when new but over time it starts to dry out, shrink, crack and separate from the siding or the flange and then leak. This sounds like what you are talking about as it does get black and dirty.
These cracks start showing up more frequently around approximately between year 8 and 10. As time goes on, they keep getting more and more and larger. Where the camper is stored and how it is used helps slow down this process or speed it up. If the camper lives outside all the time during use and storage, this is the hardest on the camper. The Sun aggravates the problem by advancing the shrinking which then leads to cracks and winter with snow and ice on the camper uncovered can seep in the cracks.
Water intrusion into the camper most times happens from one or both areas of sealants failing. The roof is often the largest entry point if the owner does not know to care for the roof and that by misfortune does not do anything with it as they see no leaks inside. However slow leaks into the wall will start and create heavy damage. Once the damage gets large enough or the leak big enough, then water will show up inside. This can be 1 or several years later after the leak started.
Putty tape sealant failures come from water being beat into the cracked areas and working it's way into the walls. Towing in the rain can create the beating water force to push the water through the crack if is the crack has advanced all the way to the inside. Some cracks stop short of full width of the sealed flange, others go all the way to the inside. Water pouring and beating off the roof or a gutter spout into a corner molding, cargo door, window or other sealing flange can cause the same problem if the camper was never towed.
I'm expanded on these issues to try and help educate folks on how a camper leaks comes about. There are many other ways as well, cargo doors not sealing, entry doors, windows etc. not sealing, however the sealant leaks are the most notorious and hardest to even see or know about unless someone explains them to you.
To your direct question, there are sort of 2 ways to help and or cure putty tape flange seal issues.
1. For a temporary patch,
(temporary meaning a few years worth) you can apply Dicor "non leveling" chalk to the exposed edge of the putty tape after you have thoroughly cleaned the exposed putty tape. While this will help seal the crack, the damage is still under the flange of the item being sealed. The crack is still there just the Dicor is holding back the water from getting to it. In time the putty tape will still shrink and crack and it can pull apart the Dicor which also has a limited life. But this method does help, just it has a limited life. We suggest this method as Dicor can be removed cleanly if needed when a full repair is needed. We suggest to not use silicone in this area as it does not last with the flexing of the camper while towing and in a year or more separate from the moving joint and leave a crack. It then is very hard to get it all off and do another repair with the correct caulk as silicone is so slippery nothing will stick to it. This post can help show one method which works when applying the non leveling, (non sag) Dicor. http://www.sunlineclub.com/forums/f7...ons-17713.html
To help clarify, there are 2 types of Dicor caulk for different locations. https://dicorproducts.com/product/ep...ealants/#specs
For the roof area which is horizontal, use Dicor Self-Leveling sealant (white) part number 501 LSW . This flows nicely to create a smooth caulked area.
For vertical areas like corner moldings, entry doors, cargo doors etc all the way around, use Dicor Non-Sag (for vertical surfaces) (white) part number 551 LSW. This sticks in place where you put it and will not run when used vertically.
2. For the long term repair of cracked putty tape.
This method requires the most work however it creates a much longer sound repair with a higher rate of success. It is to remove the window, corner molding, door etc. from the camper, scrap off all the old putty tape, clean up the camper and the flanges and reinstall using high quality butyl sealing tape. Then apply Dicor non leveling caulk as a secondary seal to keep dirt off the butyl and create a secondary seal. These posts below will help on how to do the flanges and the link above shows how to do the Dicor non leveling caulk.
This link will drop you in the middle of a roof and wall repair showing how to work the corner moldings and cargo doors. The rest of this thread may help show and explain how a perfectly clean camper on the inside had a fairly large amount of water damage inside the walls and ceiling from the prior owner possibly not knowing and not doing proper roof maintenance.
This link will drop you into the middle of a camper frame repair thread but shows how the putty tape has cracked and separated working it's way to an opening to the inside of the camper along with some installation tips http://www.sunlineclub.com/forums/f7...tml#post136157
This thread can help show you how these areas left unchecked will leak into the camper. This has both roof sealant failures and putty tape failures to the inside on a camper that was stored outside and not used for the last 7 years.
By now you may be thinking, WOW is it really this bad? It can get to this if left unchecked. The key is knowing how it happens to help prevent it from happening and if it does happen, what to do about it. I myself want to keep our 2004 T310SR camper a good long time yet. I started a while ago replacing the putty tape flanges that were in the worst shape with new butyl tape and then doing the Dicor secondary seal. I am about 50 to 60% converted. It is a large job to do them all at once, so I did the worst looking ones first. This winter I will do another slug of them.
In 2010 I took action to help stop the roof sealant cracks from forming over the winter. The camper lived outside then and in an area of snow and ice. During the winter a large crack in the roof sealant can create a big problem as you can not see it with a foot of snow on top. As the snow melts, if the crack has advanced to the point a water entry point is created, then a leak inside starts. This method I did is still in excellent condition today and I expect it to be for many years to come. The new barn came too and that really helps but the Eternabond just removes all the caulking issues on the roof. http://www.sunlineclub.com/forums/f7...ics-11610.html
If you are thinking of doing the Eternabond in the future, I would recommend you get a moisture meter and verify the camper is totally dry in the walls and ceiling before doing this. Also time suggest if you are going to pull the corner moldings to remove the putty tape, do it before the E bonding. Means you do not disturb the E bond with the molding later. This post may help on the moisture meter http://www.sunlineclub.com/forums/f7...per-17613.html
Hope this helps and let us know if you need more help