Not sure how the newer buses are on gas, but a buddy of mine from FMX days had a older bus of that size but it was and 1988, towed his ramp and put his bikes and a airmattress inside.... Was a pretty bad ass gig except it got about 4.5mpg though it was a gas motor.
The only way I would get one is if it had the 7.3L Ford diesel in it. It's a very common motor as it is found in many other vehicles as well. Reliability is good and parts are easy to find. My main concern is the transmission and how it reacts to having weight behind it.