How you go about it depends a lot on where you live. In Michigan we just paid for the bus and drove it home with no plates (which is legal) and with no insurance (which probably was not legal).
When we transferred the title at the Secretary of State we just had to sign an affidavit claiming we'd converted it into a motorhome and had installed a bed,toilet and cooking area. At the time we'd only had it a week and it was most definitely not converted.
It helped that I had reciepts for some of those things, and forged reciepts for the rest.
Let us know where you live and maybe we can be a bit more specific.