An update. The rear section of the floor...

I did that this morning and took this picture part way through. The frame spaces have been infilled with tight fitting 2" polyisocyanurate. Some of the frame cross members are channel, some are 2x4 rectangular tube. Where there was a channel the foam was fitted into the C-section. I used pieces of 2x4 ripped to fit tightly under the polyiso in the channel. Screws and construction adhesive was used to hold the wood in place. Other section were screwed and glued to the other cross members places to support the polyiso up flush with the frame top.
Then a second layer of 2" polyiso was cut and fitted to fir on top of the frame. Polyiso is rated to be able to support 20 PSI under a floor or under concrete; my cals indicate there should be no issues here. Then the 3/4 plywood that the trailer came with was refitted and screwed down with new longer screws. It was cut sort of loosey goosey, but I'll fill and cover those gaps when more insulation is applied to the inner side of the walls. For the curious let it be noted I cut the foam on the table saw or with a hand held circular saw finishing the cuts with a knife as necessary.
I'm working my way forward to the front. The front will have a 1" layer of foam on top of the frame with the 3/4 plywood on top of that. The rear section will be under the bed so there will be no step up/down to trip over. It is a compromise for headroom where we will be able to stand. The floor insulation will be thicker under the fridge, shower and the counter space in the front left of the vee.