I had a leak in the same location in our 2003 Minnie. I discovered the leak when crawling around in the cab over with the mattress off. My thumb went right through the top layer on the floor into sopping wet rotted wood. Not a good feeling! At first I thought the leak was from the front window or the clearance lights, but finally discovered the actual location by pushing in on the sidewall just forward of the Winnebago sticker. When I pushed, the seam opened up. In my case the L-shaped metal trim just barely met the bottom edge of the sidewall so squirting in some caulk wasn't going to get a seal. I was able to stop the leak by applying a heavy bead of Dicor just above and onto the trim strip. I scrape off and reapply this bead at least yearly as flexing in the cab over eventually opens cracks in the bead. With only one exception, there have been no more leaks.
My leak area
My leak repair - not pretty, but effective
The next issue was getting the floor dried out. The floor forward of the metal beam that runs across from side to side is made up of a sandwich of the outer fiberglass skin, a thin layer of plywood, a layer of styrofoam, another thin plywood layer and the inside decorative floor. Around the outer edges on all 4 sides are solid wood boards the thickness of the styrofoam and about 2-1/2 inches wide. The impervious fiberglass layer and the inner decorative floor layer pretty much seal the wood from the air, meaning that once wet, it can't dry out. In my case, I cut away the decorative layer where the floor was spongy and pulled out the rotten wood. I left the area open so that the remaining wood that felt solid could dry out. It's been 6 years and I haven't seen the need to do anything more. I put some clear plexiglass over the opening in the floor to protect the mattress.
After removing rotted wood
The rot went about 1/2 way across, the rest was solid
Plexiglass covering area where wood was removed
2003 Winnebago Minnie 24F - Ford E-450๐