I managed to box and seal off the wheel-wells after running a pair of 1" plastic pipes through them for potential plumbing or electrical lines and filled the rest with foam insulation.
It was going good, in a few days most of the walls were up and I hadn't yet blown the speakers on my little radio.
To give you a better sense of how things were moving along, here's a sequence:
The shop was at the rear of the bus, we built the bathroom next. It included both a toilet and bath area that could be easily connected or separated by a free swinging door. Here's a picture of the back wall and toiletry closet being roughed-in.
Next, the front wall of the toilet room. back wall of the closet is framed.
I chose 2x3's for the framing of the interior walls as they were cheaper, smaller and lighter than 2x4's without sacrificing a ton of strength. Since the interior walls wouldn't bear much load and were clad in ply, they didn't demand standard framing, so I saved money and space where I could.
Once the walls were in place, it was time to focus on a heat source.