I also had a leak I couldn't find, luckily I have a great RV man. He found the piece of trim near the bottom of the slide had a small gap in the caulking. He took the piece off and we found some of the screws were rusted. He resealed the trim piece and attached it with more screws. No more leaks.
I have not put any sealant yet, have been too busy and it's raining every time I have time, I'll get the sealant put on when the weather clears up, it says it takes a few days to cure totally, I don't want to put it on there and then have it rained on, I don't know how it will react to that.