We purchased a 1986 25' AS a couple of months ago, knowing the floor was damaged. Previously we had a 1979 25' AS, and found that we just had to have another one. The trailer is: front Gaucho, center kitchen/bath, and rear twin.
The new to us 25 footer had 3/4" OSB subfloor. And it had been left unattended for 3 years. Both the front and back have leaks. The last 2' of the rear steel frame was poor, sistering steel straighten that out (not caused by the classic flashing issue). I also found a 32" subfloor span in the front, resolved by adding 2- 2X3" rectangular cross bars, to prove 12" OC subfloor support.
With the water damage we removed the front and back OSB subfloor and left the middle. Replaced the insulation with 2" pink panels. New 3/4" DF plywood, on about 14' of the total interior length. On top of that a 5/8" DF plywood. The plywood is screwed to the frame. Glue was also used on top of the vintage OSB. The 5/8" plywood adds 80 pounds to the trailer, but also stiffens the floor to a degree that matches the requirements of the flooring manufacture.
The last step is to level the floor with a material made to do this job. I made sure the new plywood was level across the large spans but wherever there is a seam you can be sure the seam will not match.
Lots of folks are talking about selecting new flooring. In our case Allure works for us, we will be moderate users with low stress on the flooring. We also chose the Traffic version which has a lot more character, slate appearance.
Lessons learned: Cutting the Allure Traffic is .150" thick which allows for more surface character however, the heavier gauge does not cut well. It is not hard to do, it just isn't efficient. It takes 5-6 passes with a utility knife to get far enough though so you can bend the 12X36" tile and then cut the back side. A better cutting method would help here.
My trailer internal layout is not very square, the kitchen and bathroom walls where they meet the floor wander on odd angles. For carpet no big deal but for flooring the need to fit all of the exterior pieces is time consuming. Even though we were able to do half of the flooring install in 4 hours, 3/4th of the time was fitting. For that reason you need to have a couple of different size compasses, 6" and 12" is what I used.
Use a plunging saw to cut clearance for the new flooring. A plunging saw has a vibrating blade that you can push the saw straight into wood. It is commonly used in flooring installation to cut clearance in trim and door frames for the flooring. I used it in over 12 locations for this install. The saw is small battery operated, I think it was well under a hundred buck to purchase.
Pictures: the first two show the new underlayment with leveler over the seams and low spots, the third image is 1/2 of the Allure installed, last is bathroom to the front complete.