My floor is a built up homemade SIP, if you will, 5 mm top and bottom, 2x2 (1-1/2 actual) cedar rim frame with 1x2 (3/4 x 1-1/2 actual) xmbr's on edge. (Due to trying to maximize plywood yield, and the fact that I chose to build 64 inches wide, I ended up with more seams, and therefore more xmbr's and blocking than might otherwise be needed. Probably compromised the concept of a SIP somewhat by interrupting the unity of the panels using simple butt joints over the frame members.) The frame is fully biscuit joined and glued with TB2. The voids were filled with foam board glued in with 3M 30NF "Green" (water based) contact adhesive 100/ct coverage on both sides.
I prefinished the floor prior to assembly, and have kept it covered with rosin paper during the build. During the bulk of the heavy construction (before the front wall went on) I kept a partial sheet of 1/2 ply (to become my galley counter later) laid over the rosin paper so that I could step in, stand, walk on and work with out concern. Once that came out there have been a few occasions where I have had to kneel, and although I did this cautiously, I never really felt like I was doing any harm; only felt the slightest of give, kind of like walking on that padded Pergo flooring only less so. Once the rosin paper comes out and the mattress goes in I won't think twice about it.
As far as rock damage is concerned (I assume we are talking general road debris here, and not off road rock crawling), I think that is more of an issue for the forward facing surfaces, the front wall and cabin corners, and the tongue box. Most people report that the undersides of their cabins require very little maintenance, if any at all. Some people don't even skin the underside, just fasten insulation with long screws and fender washers.
My suggestion of using 1/2 inch ply was for if you were doing a simply single panel floor. In fact, I would be tempted to go down to 3/8, depending on wall attachment method. However, if you are going with a SIP or built up floor, 1/4 inch top and bottom is more than enough, IMO. We are talking TD, not standie, right?
One last thing, somewhat unrelated, because I will be taking TPCE on extended forest service road trips, with potential for washout conditions, boulder fields, fallen trees, etc. I purposely built my cabin to sit fully on top of the trailer frame with no wall skirts or edges hanging down. If anything bottoms out it will be steel first. Most people like to cover the frame by extending the walls down, but on a canvas wrapped outer skin it is better, IMO, to pinch the bottom of the canvas between the box and frame.