I used an adjustable pressure regulator from an acetylene tank, and started low, but ended up using as much pressure as i could get from the propane cylinder. I used 3/8" id rubber hose and a 3/4" solenoid valve that would open under WOT when the system was armed. The hose was plumbed into my air intake after the filter but before the turbo. Int dt360 all warmed up. I did a few runs up a steep grade with and without propane. It's one of the tallest points in SE michigan.
If you used a different sort of propane bottle you could allow propane to leave the bottle as a liquid then expand into a gas in some sort of expansion chamber/coils. i don't think standard bbq 20 pound tanks will allow liquid propane to escape without modification, correct me if i'm wrong though.
It's not too difficult to mount a 100 pound propane cylinder inside your bus (although prob not legal) The best idea, and probably within nfpa guidelines would be to make a ventilated exterior cabinet where you load/store the propane cylinder but have a small access door inside your skoolie to open and close the valve. Like ambulances do with large 0xygen bottles.
here is the outside compartment in an ambulance:
and the small access door inside the ambulance to allow you to turn the valve off/on