The biggest 'problem' for fuel mileage is the size of the hole you are trying to cut in the air against a 60 mph wind.
Weight has an impact upon fuel mileage, but much less than bad aerodynamics of almost all RVs, until you start hitting grades.
I have a gas Hemi in my Ram 2500, it was $11,000 less than the diesel. I give up some on fuel mileage, some on range which is a bigger issue (though the tank has yet to come out to be smaller than my bladder).
I can pull my 8,400 lb trailer up a 16% grade just fine - but I have to do it at 4,700 rpm and that really sucks up the gallons fast.
My friend with the 40' Monaco DP can go up the same grade at less than 2,000 rpm, not even taking a big hit on fuel usage.
On the relatively level road, his two foot higher rig (16 sq ft more of wind resistance) uses about the same amount of fuel $ wise as I do if diesel is about 35 cents per gallon more than gasoline.
That said, I will get a diesel next time if the budget works out.
The biggest problem as I said is bad aerodynamics. Very few RVs have efficient aero. Flat rears really add to the drag load. Nice tall square interiors hurt. Many rigs now have some better aero front ends - but if you are pulling a trailer - your TV lack of aero really hurts.
The gap between the TV and trailer or Class A/C and toad - is another big hit. Another thing is how the bottom of the TV, trailer, Class A are setup. A smooth bottom with no projections is best. But we have all these levelers, sewer connections, battery boxes, propane tanks, etc.
Going on the road, you must plan on at least spending 50 cents per mile just for transport, some folks need to budget higher.
For 2014, my towing fuel cost was 37.783 cents per mile for 6,564.5 towing miles. That does not include the cost of an approx. 4,000 non-towing miles, three oil changes @ about $120 each, a $45 safety inspection, nor the new set of tires I'll need about next April.
(We kept miles on the truck down by having my wife bring her mini-van to our hosting assignments - and using that for five months of local driving. We would have put an additional 5,000-6,000 miles on the truck without it.)
Another huge factor is HOW you drive. Two people driving identical rigs can have a 15-20% difference in fuel mileage.
I can take US-90 from San Antonio to Fort Davis - about 420 miles - and average 9.5 to 9.7 per mile by using my momentum - faster down hills, slower up hills, watching the RPM to keep the engine from down shifting as much.
I make the same run on I-10 which is 20 miles shorter distance, and will be very lucky to get 8.0 mpg, more likely 7.7 or so. Because I can't slow down to 40 climbing a hill on I-10.
Running at a steady 65 on the freeway will normally cost you 20% more fuel than running at a steady 60.
But the bottom line is still that it is going to be expensive - 8-10 mpg at the best - 50-75 cents per mile traveling.
