Your repairman is a jerk, not all Travel Trailers suffer water damage. I have one that is 42 years old and when I pulled up the floor sheathing, it was like brand new.
You will need to find out how your floor is built. If there is a deteriorated area where everything below is visible, that's a good spot to start. Some Jayco later 2000 trailers used a composite floor consisting of a luan ply sheet, an inner sandwich of foam, and a top layer of OSB or luan ply sheet. If you have a floor of that type, it will be a bit more work to repair. Other trailers have a joist system and the flooring sheets are attached to the joists.
All trailer floors I've repaired had the flooring attached first, underneath the walls. In my Starcraft, the walls were attached on top of the floor covering (carpet), too. That's pretty typical. If you are replacing whole sections of flooring, adjacent to walls, you'll have to decide if you want to detach the walls and siding to place the new sheets underneath or cut the existing flooring a comfortable distance away from the wall and replace to that point. When I did my floor, I only had a few feet of bad sheeting underneath walls and was able to cut all the fasteners in the wall, chop out the bad floor sheet, and push the new partial sheet underneath. The rest of the floor was cut back about 12" from the wall, a new support was fastened to the joists along that cut line, and the new flooring fastened into the support and joists.
Replacement in progress:

Floor as it exists today with laminate flooring:

Good luck with the floor!