Eventually

Get a ladder out and check the caulking on the roof where it meets the side wall especially at the corners. Remember there is a lot of sway while driving down the road that is magnified at the top of your slide even when stowed. Water has a funny way of finding it way down to its exit, it may stop at the floor unseen then puddle and wick up the wall seeming like it entered lower than it really has.

Trailer hasn't been moved in over a month. The slide has been out the whole time. As they say looks like it's been leaking a while from the signs. The floor is not soft but not far from it since the underside is swollen somewhat. No water anywhere else so it seems to be the slide. Does the tarp like material on the bottom keep water in or out? When extended should the slide tip toward the trailer or away or neither.
I can do the furnace blower! thanks for the idea!

Mine had intrusion along the seams near the bottom. I eternabonded those suckers after shoring up the structure.

Did you bring the slide in while it was raining or shortly after a rain? Water on top the slide will come in with the slide.

I'd suggest to test the whole trailer. There may be the possibility that there are other failed sealant spots. I use a 1/2 HP furnace blower sitting on a step ladder, sealed into a window opening with cardboard/plywood and tape to DIY the pressure test.

The squirrel cage pushes about 1000-1500 CFM. With even the simplest blocking of the built in egress points like vent openings, windows, furnace ducts, etc, I'm able to develop 1" rise in water column on my DIY manometer. I could block off nothing inside and the blower would give enough rise in pressure to fulfill the testing.

Others have used leaf blowers, rug driers, and some even say their vent fans will do it. A lot depends on area and the extent of the seal of the trailer.

Any chance the tires are actually throwing water up in that area? Might be a seal of some sort. Have you driven through rain recently?