Rotten floor

I have a horse trailer, similar but different from RV. But, have been having an issue now for 2 winters with wet floor inside the door. I have been checking seals and rivet joints on the roof, looking at the door seal and underneath. Horse trailers are built different with an aluminum roof and floor, but the carpet was wet and causing other issues. Anyway, I was waxing the trailer this past week and while up on a ladder I could look over the top of the window on the door. The seal was perfect all the way around, but had decayed right over the center of the window on the top. I am certain that was the cause of my leak. I would suggest checking everything, but look closely at the seal around the window on the door or any other windows that water could run to that spot.

It typically comes from leaky seals on the roof. Take high resolution pictures of the roof and post them here, or find a shop to inspect it for you.

I looked under the trailer where the floor is spongy but it’s a completely covered underbelly.

On our first trailer we had a soft floor develop. How it happened.... goes back to a week after we got it. I noticed one of the tires didn't match the others. It was actually a lower weight rating. And the floor that rotted was directly behind that wheel.
My guess is that the delivery driver blew out a tire on the trip from the factory. That tire tore up the fabric that kept water from shooting up onto the underside of the wood floor when towing in the rain. 4 years later I was replacing floor.
So after that long story... Check under the trailer for problems also.

Straight up if it's spongy it is trashed. You will have to pull back the flooring, remove and cut back to good wood, screw down new ply over the frame and then glue the floor back down. Trailers use chipboard for the floors which is even less durable than OSB. I do repairs with CDX.

While the floor is open use a coat hanger to reach all the insulation in the frame bay to remove it. It will be soaked with water. Let the bay dry out for a day or two before putting new insulation in and then fixing the floor as above.

I’ll check that. Thank you!

Hmmm I would inspect the roof promptly.
Spongy floors never end well.

If just inside entry door, this is a common spot for leaks. Many doors do not seal well, even high up, and the water runs down, comes inside, possibly under the vinyl flooring. Another possible source of leaks, also common, is the window in the door. Water can get inside the door, from a leaking window, and come out the door bottom, on the inside.

Jerry