Several years ago I turned a similar pop-up into a utility trailer and just last year repaired damage, similar to what you have, in friend’s travel trailer. You would be correct that the walls sit on top of the floor. The floor is attached, likely by screws, to the metal frame of trailer. Just look underneath you will get good idea what’s going on. If the floor is damaged there is a high probably the walls in that area, have similar damage too. If you do need to get into wall repair I would just replace the damaged area of the floor. Trust me you will want to repair the walls if there is rot. When I took apart my pop-up, there was similar damage and once the interior cabinets and such were removed the walls came off way too easy.

Don’t let the wall repair scare you, the construction of these trailers is amazingly simple. Since you obviously need to fix a leak, probably right at the corner piece, I would just remove the exterior siding on the two sides, for easy access for working. The siding comes off real easy once you remove the aluminum trim pieces, which are screwed in with hex head screws about every 4 to 6 inches. Be prepared to buy some new screws for the reassembly, because several will likely be rusted beyond recognition. The siding will be stapled to the wall frame and those can be pulled with end nippers. When you do any frame work repair don’t use all purpose screws (AKA: drywall screws)… Nail everything! Those screws are too brittle and will snap in the twisting and bouncing of the trailer as it’s going down the road.

Once you have frame repair done the siding goes back on just as easy as it came off, but you will likely need another pair of hands to help you get each piece lined up so it locks into the previous. When you put the corner pieces back on be sure to clean all old sealer off and use a polybutle (sp?) calk in its place, and this is not the place to use it sparingly.