Well, I've determined that the gasket around the ramp door was not nearly as tight as it appeared. Apparently, water would come in every time we drove through a rain storm.
I've also discovered that the rubber floor goes at least to the outside of the side walls, but it stops at the edge of the door frame for the the ramp door. The caulking they used has some very small gaps that allowed the water to run under the back edge.
So, I will make a cut on each side parallel to the side walls, starting at the ramp door frame. Then, I'll peel back the rubber floor until I get it past the damage.
That's the plan, anyway.
2006 T-3075 Toy Hauler
2006 Dodge Ram diesel 2500 4-door