I worked for a shuttle bus company, and we would put 200 - 350 miles a shift on the E-350 vans. Don't know exactly the mileage on the E-450 shuttle bus, 20 passenger ones got, but mainly they are driven hard and not put away from 6 am to 10 or 11 pm. So many that they are selling are in the 440,000 mile range. At 350,000 miles, they are not even thinking of selling one.
You can do a LOT to improve your RV. First thing is to have the distributor rebuilt. This is where I had mine rebuilt back in 1992, and it made the 460" engine in my F-350 come alive!!
http://local.yahoo.com/info-21034341-clutches-unlimited-midway-cityThe engine in your class C is really strong, and was also put into my 97 F-53 equipped motorhome, I have a 17,000 GVWR and tow a 4,200 pound car. I will take the grapevine 6% grade at 45 MPH in second gear, 4,000 RPM. I have no problem in the mountains.
If you are overheating, then consider a larger transmission fluid cooler, and wash the radiator. Relace the fan clutch if the RV is overheating, and you do not hear it roar to life every once in a while on the mountain passes.
You can make the 460" engine pass anything but a gas station, or make it get 8 or 9 MPG in a smaller class C like yours. If you set it up for 92 octane gas, you will get better mileage, but at a cost. You will be advancing the timing enough to make lots of horsepower, and it will make it efficiently, but will start to ping on lesser gasoline, so you will HAVE To stick with Cheveron or Mobil or Texaco 91 - 92 octane gas. As a result of the improved timing, expect gas mileage to go up about 1 MPG, more or less.
If you decide you still want to run cheap gas, set the timing back a little, and go back to the cheap stuff, and get 1 MPG less. So the differance is 8 MPG or 9 MPG, and over 4,000 miles it can be either 500 gallons of gas or 440 gallons at 5% more cost. I would say save the 60 gallons, it will be a increase of about 10% in MPG.
For now, STOP buying cheap gas, go to stuff that has as little ethonol as possible, and higher octane. If you are running 85 octane (they sell that as "Regular" in Colorado) change it up a couple of grades. Enjoy Colorado, it is a wonderful place to visit, I want to go back there sometime.
The only real advantage of a used shuttle bus is if you can find a 2003 and earlier 7.3L diesel one. They are known to go upwards of 400,000 miles without a engine replacement, though the transmission is not good for that many miles. Check Craigslist.com in Riverside CA, they have had a few used shuttle busses for sale for the past 4 months. Old city fleet that was replaced with newer units that make less smog.
Fred.