I have an old 2 valve Triton v-10 in my 1999, 36 ft Tropical MH. If conditions are right it gets surprisingly good gas mileage for its size. On flat level interstate, with empty tanks, without my toad it gets up to 10 mpg at 55-60 mph. But that is the best case scenario. Everything goes downhill from there, but not too rapidly as long as I stay out of the mountains. I usually travel with about 50-100 gallons of water and a toad which returns a best of 9mpg on the flat and level. Of course if there's a lot of stop and go or hill climbing I'm lucky to get 7-8.
The way I control my fuel expenses is to go 200-500 hundred miles to my destination and stay put for a a couple weeks to a month, using my 30mpg toad to explore the area, shop, etc. - the 2.4l Chevy Cobalt that used to be my tow vehicle when I had my Aliner. Not only is gas more budgetable that way but I've found RV parks and campgrounds are more affordable at least a week at a time. Monthly CG fees are best when available. COE parks are my favorite so far. They are beautiful, affordable, underutilized and some are even free. I budget $350/mo for my campsite and $90/mo for electricity. So far I've been able to stay well within that budget with the help of Passport America and the America the Beautiful Senior Discount. If I ever go over, such as in a couple months when I'll be visiting family in California, I'll boondock some and use county parks to stretch my budget. I'm staying in a super nice county park in western NC now, on a beautiful lake with full hookups and great wifi for only $100/wk. Life is good.
Also it's not just mpg you must consider, but fuel prices. Recently in the southeast I've been paying between $2.10 and $2.50/gallon for regular gas. Add almost dollar per gallon to that in some parts of California. Gas buddy is your buddy. If I had a diesel I would get a little better fuel mileage but spend maybe 50 cents more on average per gallon. Plus oil, fuel filter changes and other maintenance/repairs on a diesel are far more expensive than a gas engine, taking back the savings at the pump. Of course a diesel has other advantages, but overall cost/mile is not one. At least it wasn't for me when I had one.
Chip