Teardrops n Tiny Travel Trailers

If you can find the right glue and a way to clamp the PVC panels while the glue sets, I think that would be fine.

I'm not sure PMF first gives any benefit. I can tell you from experience that it is not an adequate barrier to solvents that would attack foam in a PVC adhesive.

I did 2x2 cedar around the perimeter of my floor frame with 1x2 intermediates. This gave more meat to attach walls. (I also had a centerline row of blocking, but that was more just to back up seams because my skin panels were made up of smaller pieces just butt joined, not scarfed.) Even still, at 1-1/2 thick plus 2x5mm, 64 wide and something like 9-1/2ft long my floor weighs about the same as an equivalent sheet of 3/4 ply (single thickness), but is extremely more rigid. I could pick up one corner pivoting on the far diagonal corner and not perceive any sag or flex. Very rigid. Don't forget, your fwd bulkhead and side walls will hold the floor up. The box is the deepest "beam" structure in your build, and has more strength in it than a 2x4 placed under the leading edge of your floor. Build nice strong "composite" (well glued) walls and attach them well (construction adhesive and screws) to your floor.