If you are really interested in higher MPG, you might consier one of the Sprinter based RV's. However they tend to be underpowered, and the transmissions on my work vans tend to go out around 120,000 miles. The company I worked for owned about 8 of the Sprinter work vans, all the transmissions went out before they reached 135,000 miles.
For a lower cost unit, to purchase, insure, and drive, including things like yearly oil changes, and fuel filters once every couple of years, consider a gas engine RV, and it will only get around 7 - 10 MPG. So you drive it what? 5000 miles a year? 7 MPG is 700 gallons of gas, 10 MPG is 500, something like a sprinter will still be getting 14 MPG or 350 gallons of more expensive fuel. So you spend $2,800 a year on one, or 2,000 at 10 MPG, or 1,550 at 14 MPG on diesel.
The sprinter will have limite space inside, and will cost more to insure, oil changes, yearly fuel filters, and more expensive to take to any dealership for anything.
The Ford or GMC van based RV or class A will have the same engine as the pickups, and will cost a reasonable amount to get a oil change, insurance should be a lot less if you are only getting normal coverage, and the unit cost is much lower. (insurance for a $35,000 RV will be more than a 15,000 RV that is a couple years older).
What I have is a 97 Fleetwood Bounder, 30E, that has a huge bathroom, no slide outs, and gets around 7 MPG. The last trip I was towing my very light 1,500 pound Manx buggy, got 8.8 MPG using 92 octane gas, and driving 55 MPH. Going slower really helps. It is 30'11" long and has a unusually large 100 gallon fresh water tank, so camping out in the boondocks for a week is possible, even with daily showers.
I used to have a class C, but it could not even tow a small trailer (back in 1985, the chassis was not steel frame on the GMC's like in the 90's). So I had to move up to a class A to have more towing ability, and a larger engine.
Any class C from Ford will have the V10 engine with about 300 HP that is 97 or newer. All are fuel injected from about 88 and newer, but 88 - 96 used the very reliable 460" V8. They have full frames, and depending on how the rear of the chassis was extended might be able to tow a good sized load.
Good luck, I need to sign off and go camping!
Fred.