I had a ~6'X4' section I had to replace. https://www.irv2.com/forums/f115/new...ks-564571.html
On my coach the manufacturer only used a vapor barrier to protect the underside of the OSB plywood to protect against the road elements, I had tow layers of ~5/8" OSB. I was able to salvage this and made an attempt to adhere it the underside of the new plywood I put down by spreading liquid nail and then using a foam roller from underneath once the new plywood was dropped into place. Hope it works.
I decided to use a 3/4" piece of treated plywood for my first layer. I used exterior primer paint on all sides, painted the bottom and sides first, then laid it down and pained the top. I did put some additional plates underneath at the seams to tie the pieces together. The 3/4" is stronger then 5/8" for the spans between the metal "joists" underneath.
I then had to make up the difference with a a different sheet and had a piece of exterior siding that was "close enough" to use and this has a 50 year warranty so it is very durable. It is a very dense product also.
I then put down Pergo LVP flooring.