My daughter did this in Idaho earlier this year. Idaho RV code is more lengthy and detailed than Michigan's. She had to install four things from a list of what they consider to be necessary in an RV. So she bought a full size stove with an oven and put it in there, and fixed up a bathroom area, installed the beds, and bought a cooler to hold food. Really, not much. The stove was the biggest and most expensive thing. Your state code just specifies, "permanently installed cooking and sleeping facilities" ... so do that, and see if they'll register it as an RV. When my daughter registered hers, someone went out to the bus and looked at it, to make sure it qualified.
The most difficult thing they've done to the bus so far was to take up the old flooring, coat the metal with rustoleum since it was somewhat rusted, and then install wood sub-flooring over that. Now they are living in it and have built bunk-beds and a kitchen cabinet. There's a lot left to do, but at least they're happy to be out of a sticks and bricks rental.
