While fuel costs are probably one of the largest expenses with Rving, it's still far better than plane, car, hotel or any combination of those. A 32' gasser is probably about the same price as a truck TT combo and while the MH may get a little bit worse mileage than the truck pulling the TT (maybe not) the toad you can bring along mitigates this. I think you are splitting hairs. The difference even if it were 5 mpg difference over a 1,000 mile trip is about $100 in fuel expense. (1000 miles @ 8 MPG @2.29 fuel = $286. the same @13 mpg is $176).
The benefits of a MH towing a car vs a truck towing a TT are huge. It's much easier to hook up a toad than a TT, much easier to set up camp in a MH, all the benefits of actually using the MH while travelling vs waiting until you get to where you are going and set up. etc..
But why would you assume you'd get so low mileage? You'll do much better than that.