First let me start off by saying that so many people, myself included, start off by wanting to build a slide in or removable cabin; but then we find that these projects get a lot bigger and more complicated than we initially thought, and quickly realize that the practicality of doing so is just not that easy. The idea is that it will be convenient to switch back and forth without a lot of extra complication and effort, but it just doesn't work out that way. Yes it has been done. Yes you could make it relatively "easy", but there are design compromises that most people aren't willing to or can't logistically overcome. You have to keep the wiring, lighting and fenders separate; and you can't easily use a tongue box for your electronics or house battery.
That said, let's think about your question. For a removable cabin you need to think about how you will remove and store it.
A super light build, like an empty WAZAT can be lifted by two hardy people, but let's just assume that your build will take a little more effort; most don't end up that small and light, especially not those with basement storage. My guess is that you don't want to have to remove everything out of the basement and cabin in order to make it light enough to lift by hand.
If you plan to drag the trailer out from under it, like putting a boat onto stanchions or blocking, then the scuffed surfaces under the floor need to be pretty robust. Perhaps two or three skids (long strips) of hard wood would do the trick and stand up to that kind of wear, but then those strips will create point loads where they attach to the under side of the basement floor.
If instead you plan to use some sort of lifting device, like an a-frame hoist and straps, or jacks that attach to the side of the cabin and straddle the trailer, then whatever you use on the bottom doesn't have to be so concerned with scuffing or sliding around, but you still would have to worry about point loading if you don't have a nice flat even surface to set it on while not on the trailer. And if you have budget to build an A-frame and space to store it, then you definitely have budget and space to store a separate UT instead (see what I mean?)
For me water intrusion would be the biggest concern. I would build the bottom of the camper like a box or tub with at least a plywood floor, if not the whole tub; and then add skids underneath. Then build the cabin on top of that. You wouldn't have to use super heavy plywood if you go with corner cleats and some intermediate dividers, perhaps 3/8 thk max with 2x2 corners. I suppose you could even just skin under the bottom with ply, seal with whatever, maybe glue canvas and paint, then attach the skids; then build the sides of the tub up with foam and place a couple more hardwood rub rails near the top sides of the tub where it might contact the side rails of the trailer.
Have you seen
desertmoose's build? It's not a slide out, but his design could easily be converted to one in foam and it has very practical features for sealing and access to the under cabin storage. The way he incorporated access to the front areas thru the side door openings and divided the space for organization with bulkheads that also support the cabin was genius, IMO.