Replacing a gas engine with a diesel

If a diesel is a must, and if the OP is handy with welding tools, then why not a used school or prison bus? It won't have a RVIA tag, but a skoolie would have a diesel engine, a number of them have the engine in the back so driving is quiet, and they are usually well maintained.

I've seen a couple skoolie conversions. Though they are not my style, I've seen some pretty amazing things done with them. They definitely bring the most room in a motorhome for the price.