It seems the only reason not to use a motorhome door would be looks.I would like to do a custom door, but extra money for windows and time to get windows is a concern. I'll probably end up with a motorhome door unless I can find someone to create a customer door for a bit less than the price I got already.
Is the side of your bus straight? If you put a straight edge from the bottom to the top and there is a crown at the midpint, like on MCI's, even a high dollar door will look like real bad. They will have to build out either the top, bottom or both to fit the flat door in the crowned opening. You'll have an RV door + $500 or so to jury rig it to your bus and in the end, that's what it will look like. It should cost $2000 or less to have a door made from scratch. If you can't afford it now, I would wait...or convert the air motors on your existing door to electric. That wouldn't be hard to do.
The hinge does not have to go the full length of the door.