I have gone through a lot of trouble to get my bus licenced and insured and i have found, the best way in Ontario is to get a bus that is older than 15 years. It then becomes special interest antique. You then change the paint and take off the red stop sign. This makes it licenceable but to insure it i recommend ING insurance. Since my bus was a Ford busette, I checked out the frame and it turns out to be an F350 frame. I then insure it under a 1965 Ford f350. My appraiser recommended this after I told him what happened at the insurance office. This offers great rates starting at about $75/yr with full coverage. You cannot, however have more than 8 ppl on the bus and it is not to be used to transport kids. You also do not need you bus licence. If you have been driving for a while, another great insurer for antique vehicles would be Silver Wheels.
If you have an antique bus- there isn't really any minimum requirements for being an RV but you should check up on it for sure.