We have a 30ft. fiver which was plagued with galley slide leaks. Due to leaks, I had to replace carpet/pad and install laminate flooring. Fortunately, the subfloor was ok.
3 shops worked on the leaks. I spent over $1K between them and still had the problem. Fiver was only 5 yr. old. Shops indicated they were perplexed on the slide leaks claiming it must be operator problem.
By chance, we stopped by an rv shop in Quartzite, AZ coming from Ca a few months ago. They recommended we try some new gaskets. So, what ta heck....I bought two type of gaskets (which I had been researching on line) drove home to Casa Grande, AZ. Both wife and I pulled off old perimeter gasket and replaced it. What we discovered is that, the metal stop bar on the inside of the trailer which supports the exterior gasket was bare....nothing on it. Therefore I put a "D" gasket on that along with a garage door gasket. Adjusted the position of the slide (which isn't rocket science...anyone could do it).
Guess what....now that we're in Oregon where it's raining nearly every other day....no leaks. Have been through a couple windy/rainy storms without a drip inside.
Gasket installation only took a few hours. Don't be afraid to tackle some of these projects. Do lots of research and talk with a few techs.
Best of luck!!!