Step 2: The foundation

I had some kind of strange idea about the caravan itself being detachable from the trailer.  It is, but it wouldn't really make sense to do so, unless maybe the trailer needed an upgrade or something.

Still the thing is designed to be self supporting, and just bolted down onto the trailer.  It starts with a 4'x8' sheet of exterior grade plywood, painted with an exterior grade epoxy paint.

Painted, notched to accommodate the trailer bolts (I wouldn't do that again, I'd have the trailer welded together and leave the floor intact).

I made the runners out of 2x10s with braces to keep it square.  These are glued as well as lag bolted together.  They are 10' long, 2' longer than the trailer, and the extra two feet hangs out the back.

The runners are then thru-bolted through the floor and onto the trailer itself.  I couldn't find bolts long enough so I used 3/8" stainless steel threaded rod (12" long) and bolted with lock washers on both sides.