Personal experience:
Instead of using plywood, I used XPS foam board. I have done this for both my campers. I built a frame of the size of the camper floor from simple 1x2 wood, and filled the area in between with 2" XPS (Extruded polystyrene foam) board.
Sorry, I have no pictures right now.
The stuff is pretty weatherproof and insulates quite well. It is hard enough to carry the camper. It is also inexpensive.
In the first camper, a Lance 915, the difference in floor insulation was quite noticeable.
The new camper, an Outfitter Apex, has a basement. So there is no direct effect that I could notice.