Thank you for the replies guys.
SO the foam that I intend on removing is 1 1/2" thick. If I replace it with 3/4" plywood and 3/4" hardwood, it'll be the same height as the old floor. My slide out is effed up. Basically one side where the kitchen sink is sags and drags the floor. This particular slide out has just the one track in the middle instead of one on each side so when I start tearing this thing apart hopefully I'll see what the culprit is there. I've seen videos where people make makeshift dollies with jacks on them to lift them up. I will make a new thread on it when I get to it.
My concern with the air pockets was the waterproof lining beneath the trailer. With there being gaps I would think it would flap around when pulling the trailer and damage it. If I do the plywood/hardwood route I won't have this problem. I guess I would just have to ensure I have the correct length of nail so it doesn't puncture my waterproofing.
BTW what is that stuff called? Can I kinda patch it with some flex tape or something? I don't want to cut corners... but replacing the whole thing seems like a lot of work and 80% or so of it seems just fine.
I am going to begin demoing the inside. I'll post pictures and anything else relating to this issue in this thread. I will also start another thread that documents my build as I go along. Having a really nice and unique camper has been a dream of ours for about a decade. We want to really deck it out and make it unique and beautiful. Thank you all for all of the helpful replies. I'll do my part in posting and documenting my problems and resolutions so it'll help someone else down the line.