I have seen some of the siding bulges before, not sure how large yours are and what time of year you see them. Winter, summer etc.
The one I last recall seeing was a 2007 T2499 from a Sunline friend. Theirs's too was on the non door side, up towards the top, just below the round rippled siding. This is not a result of the siding being so wide, I have had several Sunlines apart and not had this issues. I'm speculating, but it may be a late 2005/2006/2007 ish model camper thing that happens. Sunline may have changed something in the shop practices to aggravate this. Maybe we can have more members comment back if they had it and what model years.
Here is the issue as I believe is occurring. During the siding install, the shop person who was installing that piece of siding pushed it up, "too tight" into the lap joint of the sheet above it. The siding is installed from the top down, unlike a house which is bottom up. By having the sheet "too tight" thermal expansion of the long sheets without a window, door, slide molding etc. will pucker in the center and bulge out. I can very easily see this happening from the siding installs I have done. The fact the sheet is long aggravates the problem as with the longer length, the overall length expansion can be greater even thought the same sheet vertical height is the same as other tall sheets.
So that is the issue as I know of it. We always learn something more on every one of these kind of things. When I install the siding, I make sure it is up fully into the lap joint, but not over compressed to almost start a pucker from day one. Just up enough to have a full seating of the siding, then staple it on.
Now what to do with it once you have the issue. This comes down to a subjective judgment call, and seeing it compared to a straight edge up and down the siding helps in making the judgement call when to leave it alone, and when to do something with it. I have never seen a sheet pop out, the problem is not that bad. Hot sunny days can make the problem worse sometimes, but I have seen it in late fall when it is not so hot too.
An 1/8 - 1/4" pucker, I would leave, this is very common. A 1/2" - 5/8" or larger pucker gets to be, well my limit. Especially if the issue persists over a large part of the year in the non winter months.
Your thought on the fix is in line with what have done, but I caution to not over do the areas thinking all the siding has to be tight. The camper flexes a lot going down the road and over fastening can create it's on set of issues too. I would address the heavy pucker spots, and leave the mild ones.
Using the gasketed screw approach is an option. Do not settle for cheap screws though, get a quality brand as I have seen the rubber gaskets crack all up on steel sided buildings from bad gaskets.
Another option and is what I recommend, get stainless pan head screws. # 8 x 1" long will work. I use square bit drive heads, but hex head or Phillip's will work too. Take care "not" to overtighten the screw as the standard 10-8 or 304 stainless is soft in comparison to hardened zinc fasteners. The heads will pop off if you over tighten. Suggest to start the screw with a drill driver and "stop" 1/8" short of tight and do the rest by hand. A good cordless drill with a clutch on the end of a good size capacity can pop the heads real fast as it can easily overpower the screw even on a low clutch setting from the speed of the drive.
You want to, have to, find the stud and ideally the center. I can tell from pushing in on the outside as I have had walls apart a lot and know the feel. But for the first time do'er, suggest a little more investigation. If you have a stud finder, mine does work inside the camper scanning the walls. Find a window ideally in the wall you want to work on, measure from the edge of the metal window frame to the wall, to the inside wall stud centers. If you have no stud finder, then do a push/feel test on the inside wall board. You can tell the feel of a solid stud and no stud. Write down studs dimensions from the window in the area you need to work in. Then go outside, measure from the edge of the white window frame down the wall to the area you want to work in and pencil dash the siding on the stud centers. Then push the siding in the center of the sheet to feel that, to confirm you found the stud. I say to push at the sheet vertical center as at the joints, the seam is very stiff and you can't feel feel it as easy.
Pilot drill with 1/8" bit for a no. 8 screw, so this does not set up a rip location in the future from flexing while towing. The siding is thin so do not drill into the wood other then just a touch of it. You can poke the hole with an ice pick if a wood shaving did not come out to confirm you are on a stud.
Wipe the siding clean with denatured alcohol, isopropanol rubbing alcohol or Naphtha. Shoot a small dab of Geocel Proflex RV caulk, into the hole, then insert the screw, drive with drill, hand tighten. Using a soapy wet finger smooth out the Proflex goo around the screw head. You can go over the head of the screw if wanted.
This is the Proflex RV I'm talking about. I use white or bright white on the white Sunlines. https://www.geocelusa.com/product/pr...xible-sealant/
If you cannot find the Proflex RV, (it seems to be in demand right now) in this case then use the standard Proflex you can find at some hardware and lumber yards https://www.geocelusa.com/product/pr...lymer-sealant/
I suggest not to use silicone based caulk, it will not not create a long lasting bond in this application.
Hope this helps and feel free to ask more about this or anything else on your Sunny.
Happy camping this year,