Take an ice pick or something similar and probe around the edge of the inside skin and around anywhere where moisture can get in such as the door, vent pipe, shower, you name it.
But first you want to get a good piece of plywood and see what it feels like when you poke it with the pic, then when you check the trailer you'll know.
On my trailer I thought I knew where it was but when I took the shell off I found a lot more. There were areas where the floor was good but the last inch under the U channel was a mess.
"On a vintage Airstream if it aint absolutely broke DON'T TOUCH IT!!"