Our bus: 1961 GM 4106, 8v71 with v730 transmission. We're 35' long, and with most tanks pretty empty, weight just over 24,000 lbs. We tow a 2500 lb Mini Cooper behind. We typically travel at 55-60 mph if the speed limit is that high (we avoid interstates if we can.)
After 1.5 years of full timing in our bus, we average about 6.6 mpg, and that's will usually about 5-6 hrs of generator time (which is plumped off the same tank), and running the roof air off the alternator if it's warm out on a driving day.
We find that to be pretty darn good mileage for a house
Keep in mind, to a full-timer, fuel becomes part of your rent. And the beauty is, you can control how much you spend of your monthly budget on rent by varying up your pace of travel. So the MPG is useful info, but not as important as how many miles you drive. Need to trim back the costs? Find a cheap camping situation (monthly rate, workamp, find a courtesy parking situation, boondock on public lands, etc.) for a month or two and be more stationary. Want to make a major transition? Budget for it.
Here's a post we did a while back further explaining it:http://www.technomadia.com/2012/09/a-full-time-rvers-perspective-on-fuel-costs/
It includes a link to our spreadsheet calculator where you can plug in your anticipated mpg and annual miles, and will calculate your monthly fuel costs at various prices. For us, it helps us keep it all in perspective... and perhaps it can help you with this budgeting question?