I've spent 2 days slaving away on the base. I think I have enough basic support.
It is 8 ft wide and 20 ft long. It's 2" longer on the front than the original camper. 7-1/2" Longer in the rear because I extended from where the back wall used to be, to the back edge of the bumper. That gave me the total 20 ft length. The original width was 7ft-8". I widened it 2" on each side to get the 8ft width. Due to using 2x4's instead of 1x2's, the inside space will be about the same as the original camper ... just better insulated.
I still need to run some strips up the middle and re-enforce a couple of spots before covering it up.
I kept the original gray-water and black-water tanks. The black-water tank will be used for the tub. The gray-water tank will be for the sinks. We'll be using a sawdust toilet -- we've had good experience with it.
Up towards the front there are things that just don't line up. Side tabs not lined up with the metal cross-ties. Step tabs that are just wonky. I did put crosswise 2x4's centered at the 8ft and 16ft points. That's so my plywood sheets will properly secure lengthwise. This all makes for inconsistent measurements between the 2x4's. And the more expensive pressure treated 2x4's are the ones that warped the most.
I'm going to have to shave the high spots on the twisty 2x4's to level out the floor surface. I'll use my triangular toothed tree saw to do that. It's called improvise to make it correct.
My neighbor has offered to use the forks on the front of his tractor to lift the wood frame off the metal frame so I can put flashing on the bottom. Then he will put it back on the metal frame. And I can proceed from there.