Tom, 1st question....what did the dealer do to verify the leak was there?? Had a SOB TT once with the same issue. Took it to the dealer. They said they could not find it...it was not raining. My reply was...ya got a water hose?? We took the trailer and traded in on the Horn.
The only way you will ever find the leak is either with a water test from underneath or high air pressure. Well, ya could fill the trailer with water and see where it runs out but thats really not a good idea. LOL With a bright light, with the trailer dark inside, hold the light in the suspect wheel well and see if you see light anywhere. You need to crawl under it yourself and look around. Have some good caulking handy and caulk the doo-doo out of every place that looks edgy. It could a rubber seal that is out of place. Take a look at the side that does not leak and see if there is any difference. Heck water test yourself with some one inside to yell when it is leaking. Dont just blast it with water ..start at one end and move slowly across the whole area. Rain on the road at 60 MPH, blasts the wheel wells pretty good.
1 more thing, make sure that the slide bottom "bulb" seals is compressed on the side that leaks. The slide bottom may need to be adjusted in. If I think of anything else I will post it. The only I can say is that water leaks are tough. 45+ years of chasing them in cars will make you a believer.