Bruce,
Check out the link below to my web page. If you scroll down you can see pictures of the floor in a couple of different stages.
http://www.edzech.com/EZ%20Teardrop%20Camper_files/EZ%20Teardrop%20Camper.htm
My construction was pretty much the same as anyone except I haven't seen a lot of folks build all the way outside the wheels. Hope the reason for that is asthetic. My camper is not as traditional as some because it is wide, has no galley and has bunks.
There should be gobs of posts to search on (try 'floor and insulation'), and the conclusion it seemed to me was leave the foam insulation exposed without a "bottom" so that you don't create a space for moisture to collect. I also painted the underside of my floor as opposed to using some kind of tar substance which I think will be fine but I am also trying to build cheap and fast or I would have used epoxy which is probably overkill too.
Do what feels good to you. Once the 2 X frame gets glued/screwed to a 1/2" sheet of plywood and them bolted to the trailer it is pretty damn solid. I would have used 2 X 2s instead of 3s but I needed to raise the floor above the wheels.
Ed