We bought our 15footer late last summer. we only had it out twice for camping. The roof did not, and does not, leak. But we some how got involved in some severe rain downpours last fall. Holy good grief did it ever rain.
Later when we lowered the cushions to prepare for bed I noticed a small black spot on the wall. Yup, your guess is correct. it was the beginning of spreading mold.
As soon as we got home I decided to remove that entire wall panel expecting even more damage. I also found some of the internal strapping to have some mold. I removed the front window and replaced all sealant with new putty. In my opinion putty is best. Using light I looked for the smallest pinholes to show light through the wall.
One RV shop told me that the putty should be replaced every 5 years or so. I did not know that but now I see why. Removing windows is no big deal to replace putty (DO NOT USE silicone)
It turns out this leak was a long term leak. The work was time intensive but did move forward without issue. When it's your own project, time is of no concern.
The biggest problem I had was trying to find a matching
sheet of wood-grain paneling. The RV shops said this was an obsolete sheet and no one makes it anymore. I searched and searched and finally found a sheet of wood grain very close
to that at Home Depot. If you measure correctly one sheet can do the entire wall.
Everything went better than planned. The only mod I made was adding a better thermal insulation. I also found the previous owner messed up some of the internal wiring so I replaced all that. Their existing 12 volt wiring was totally screwed up so they added some 120VAC wiring and a fluorescent light all tapped together. A dog's breakfast.
I have replaced all the original exterior incandescent lights with new LED fixtures. I did the same for all the interior lights.
All our cushions were in fantastic condition so a simple shampoo cleaned up other people's cooties
New trailer type Tires and new bearings and brakes. New electric brake controller.
Then whacko winter came yuck....
Now when things warm up I will be sanding down the chassis and repainting a high gloss black and adding a new propane tank cover as well as a new spare tire cover.
Then strip down the roof of all silicone from the previous owner and use a proper roof protective coating.
A little advice: when replacing any wiring, try to solder the connections rather than crimping and seal with heat shrink. Avoid using electrical tape for any repairs that end up hidden. Any repair you do take your sweet time and do it right. make sure you research if you're not sure.
I call any of my projects "Pride Of Ownership"