I did a similar stupid thing with our previous camper. The underbelly was that tarp stuff with insulation on the inside. Water headed right to the furnace air ducts, but also to the side of the trailer and, sure enough, the insulation on the underbelly was heavy as can be, saturated with water. What to do?
I punched a couple holes in the underbelly material and let it drain and drain, and drain. Once the water slowed down to a drip, we headed home.
Once home, I had to figure out a way to dry out the insulation under the floor.
I ended up blocking the heat ducts on the floor of the camper, and actually bent one heat duct up so it would blow under the floor. I then covered the vent top side. I turned on the furnace full throttle and let it run for about 12 hours. I went through an entire bottle of propane, because the furnace never shut off. It just kept blowing hot air under the floor.
I also made a bigger cut in the underbelly so air would flow out being forced in by the furnace.
After about 12 hours, I could reach in thorugh all my holes and feel the insulation was dry. I think (If I remember), I let it blow another few hours the next day (on a second propane tank of gas now), until I felt the insulation, floor, and everything underneigh was 100% dry.
I then had the task of taping up all those holes and then fixing the air duct in the floor again (that I ended up beating up pretty bad).
In the end, it all worked out just fine. And we had the trailer another 7 years and THAT NEVER happened again!
Bottom line: make sure the underbelly of your camper is dried out. Not just the inside where the living space is.
