After riveting the sheet metal to the trailer frame, we attached the floor joist with large bolts in several places. The L-brackets we had welded to the frame provided the perfect mounting surface.
The subfloor was built on 18″ centers, and insulated with rockwool.
We sheeted on top of the joist with 3/4″ plywood, attaching it with screws and hardware glue.
This is what the van looks like when you are too stubborn to make two trips across town to the specialty insulation warehouse.
With the subfloor complete, next step is winterizing the house with a roof and some sheeting.
You can see a video of my build at https://www.youtube.com/watch?v=hYe1dT-fNQk