A 1970 bus is probably going to be a small gas engine with a stick (and not the fun to drive stick.... heavy duty long throw all the way and maybe even non-synchronized), no overdrive, 2 speed rear, no A/C, and about 3-4 MPG (seriously). Maybe a bit more since it's a short bus but still low single digits. No fun in the mountains too. Older medium duty trucks / busses are driven one way, gas pedal to the floor. They are cheap, and essentially the construction is the same on the old ones as on new busses. If you are parked (camping, whatever)most of the time, it's probably not a big deal, but my decision would hinge on usage and then condition. Lots of driving, old bus, no good. Also the chassis matters on the old stuff. NAPA carries a lot of old truck stuff, but if you are stuck with an orphan chassis, parts can be hard to find (example, some of the chassis with the wide 5 lug setup can be hard to get brake parts for). Don't get me wrong, I love old stuff but for certain purposes.
This is not to say it's a horrible deal but 10 year old busses with ~100k miles are less than $5k. They have A/C, 300 HP diesels, and automatics (if that's your thing). Front engine, rear engine. Most are larger but I've seen smaller ones. Heck, this would be a viable option and cheap (local to me... not you unfortunately.. although a plane ticket from DEN to PIA or ORD is cheap and the drive back easy...):
http://peoria.craigslist.org/cto/4681632835.htmlOver 10 MPG and as much room as you need, and a regular Ford dealer can work on it.
There are companies that specialize in used bus sales but naturally like a car dealer they are higher priced than typical CL.
Just some thoughts but I think a much newer bus might actually fit your needs a bit better.