gas mileage

7-10 MPG with my 8.1L GM engine. In the Western mountains it drops to about 6 mpg. By towing you basically double your mileage because you are getting TWO vehicles to the destination using about the same amount of fuel. :)

8-12 depending on a million factors like wind, terrain etc. I never bothered monitoring MPG. When someone asked what kind of mileage I got in our MH, I would say; "it gets great mileage for an apartment".

40' diesel pulling a 20' enclosed trailer with a Hyundai SUV inside: at the end of the trip, I showed 8.3MPG on my computer. Always traveled the speed limit.

8-ish on the interstate is probably a reasonable guess; at that length, you're not likely to be talking of a diesel motorhome.

Weight has some effect on the mileage, but not as much as you might expect for highway driving; once you're moving, it doesn't affect the rolling resistance much and affects the air resistance not one bit. It does, of course, affect the mileage when going up hills or in stop-and-go/city driving where one's speed isn't steady.

(Incidentally, most gas class C motorhomes also get around 8-ish mpg.)

Mileage also depends on the weight of the rig and not the length.
Speed is a big factor. By keeping in the 58-62 miles per hour, our rig gets the best. Mountain terrain drops mileage.
We have many miles on our rig. It has gotten as high as 12 and as low as 6. It varies about .5 MPG towing our Lil Car.

The speed makes more difference then towing or not. We get 8 or 9 unless I go 75 then about 7.

I had always heard the statement “if you are worried about the gas mileage you can’t afford it.” After having owned a class A gasser for four years and a a class A diesel pusher for six years I understand and completely agree with that statement. We get about 8 to 12 mpg towing or not, tail wind or not, gas or diesel.

8-12 mpg

It's a 'wind sail'