I have been around propane forever. It used to be a lot simpler. Now days if you don't have the right valves in the tank, they won't fill it, but they'll upgrade the valve for a nominal fee...
You might check with your local propane dealer to see what they have to say, but you may run into a snag if you permanently mount portable tanks- 20lb or 100lb. LP tanks designed to be permanently mounted to vehicles have guages, vapor ports, liquid ports, fuel level senders, emergency pop-off valves etc. Your typical grill bottle does not. I would guess the best bet for using portable tanks would be to keep them portable.
Also, the propane guy isn't going to crawl under your bus to fill'er up.
Something that might work well on a bus is camper tanks. They're 8ft long and mount over the wheel wells on full size pickups. Slide in campers fit right over the top. They're 40 gallons each. I have a pair of them between the floor joists in my shop as air compressor tanks.