Your camper has a basement, doesn't it? In that case I doubt any insulation below the lower floor will do much.
Having said that... we have made very good experience (and not even intentionally) with 2" xps foam (goes by the brand name Styrodur over here) underneath the camper floor in our old Lance 915. We needed to rise the camper to clear the tailgate opening of our Silverado and put it on the foam boards. We built a simple wood frame with the same dimensions as the camper floor, filled it in with closed cell foam boards, and hung it from the camper floor. It made a noticeable difference as far as floor insulation goes - no wonder as it basically doubled floor thickness.
Do not use open cell foam as Styrofoam as it will draw and retain water.
In a basement camper I would probably try to put in carpeting against the cold.
