Best mileage for a motorized factory-made RV, so far, in the U.S. is a B on a Sprinter van.
Because of the price of the van, and the cost of a B conversion, that may not be the best bang for your buck because the extra money you pay for it might more than cover the cost of fuel for something a lot less expensive, even over 10 years or more, depending on how much you drive. For example, at current fuel prices, 10 mpg motorhome might cost $1500-2000 a year more than a Sprinter to drive 5000 miles a year, so it could take 20-40 years to make up the higher initial cost. That's just comparing to a 8 mpg C that costs $60,000-80,000 new to $100,000 to 140,000 for a Sprinter B. A 15-16 MPG Chevy or Ford B at $20K less than a Sprinter narrows the MPG gap and further lengthens payback time.
For a lot less money you could do a conversion on a minivan that might get 18-25 mpg, or a compact van (like Transit Connect). But you have a much smaller house to live in.
Or for real bang for the buck, you could get an old front-engine diesel 26 to 30 footer that gets 10-15 mpg for under $10,000, and use the $120,000 you saved to buy fuel for the next 25 to 40 years.
If it doesn't have to be a motorhome, you can pull a small pop-up trailer behind a small pickup or minivan, and get than 16-20 mpg towing. Even buying new, you would have less than $20,000 in the RV.
Scaling down more, there are tent trailers that will tow behind compact cars and get better than 20 mpg with the combo, and better than 35 mpg when not towing. In Europe, they simply convert the MPV version of cars this size into camping vans, and enjoy more than 40 MPG (because they can get the tiny diesel engine and don't mind shifting the transmission themselves).
I'm also 66 years old, recently alone, and don't need a thirty foot motorhome that sleeps up to eight people in three sleeping areas. I've been seriously studying the alternatives (including dragging out my 30 year old tent to check its condition), and working out the economics.
Unless you have money to throw away, spending it on a $100K to $150K motorhome that gets 15-20 MPG rather than something half the price or a tenth the price that gets 8 to 16 MPG simply does not work out, on the dollars. And if you have that kind of money to throw away, a few hundred to a couple thousand dollars a year for fuel is not that big a dent in the budget, compared to the $3000 to $8000 a year fixed costs to simply own the thing.
The economics of RVing involve much more than fuel costs.
On the other hand, the psychological effect is great. I've been leaving my motorhome sitting (still costs me a heap of money whether I use it or not) and paying for motel rooms when I travel because my car goes 4-5 times as far on a gallon of gas, and my mind is stuck on "50 cents a mile to move this thing, 50 cents a mile."
I just bought our first RV this past Aug and knew up front that it wasn't going to get good mpg but like the old saying goes, you can't take it with you. Most of our trip have been within 100 miles of our home so that doesn't cost me an arm and a leg. In May we are driving to northern Ca for a week camp out and I figured it is going to cost me approx. $562.00 in fuel based on 6mpg and the cost of gas being approx. $4.50 gal.
With that being said, to get the most mpg, make sure your rig is tuned up, drive 60 to 65mph and DON'T haul all your holding tanks full. i.e. frest water. Water weighs approx 8 lbs per gallon so if you have a 50 gal fresh water tank, you are hauling 400 lbs. Get what I am saying.
I'm not 66, but I'm getting there and I'm transforming my 20 year old Dodge B-250 into a one-person RV. It can be quite nice and self-sufficient. And gas mileage 16-20 mpg. Look at Cargo Van Conversion.
My website describes the conversion of my Dodge B-250 van into a small RV. I deal with a lot of woodworking, but hope to be quite specific on solar as well.
PS: 90% of RVers inadvertenly lie about their gas mileage, even here on the forum. It's like golfers. If a guy hits one drive 280 yards in a season, but his average is 210 yards, he really thinks he drives the ball 280. RVers have the same defect when it comes to remembering/reporting their mileage.
Then of course there are the RV salesmen, need I say more?
If you look at an RV and they give you higher numbers than what I wrote above, be very skeptical. They might be telling the truth, but you just can't count on it.
LOL...yeah when my coworker bought the Thor ACE last year, he said the delaer said he would get 10mpg. I told him good luck with that. It had the same engine as my F250 has in it and my truck not pulling anything gets barely much more than that...I'. happy that during the off season that it climbs higher than 11.2 mpg! lmao
I guess I have to add my two cents, too. My wife and I started off with a Coleman tent trailer and two dogs(border collies). I towed it with a Toyota SR-5 pick-up & 5spd man. trans. Ball-park mpg was around 26mpg-if I kept the speed down below 65-70. I loved that little trailer although you heard every sound outside(including someone's child at 6:00am riding a big wheel on asphault)LOL.
I just bought a class A with the Ford V-10, and have heard anywhere from 6 to 7.5 mpg. (I haven't taken it on a long trip yet to det. mpg). I am not rich, but I'll be damned if I will let the oil co. deter me from using my RV. But I will try to save every drop I can. Some important maintenance items seemed to be mentioned alot; like tire pressure freq. checked. Clean air filters and engine properly tuned: proper viscosity or synthetic motor oil; but keeping the speed down and avoiding rapid acceleration. The danger here is driving 55mpg in S. California. Even the big rig drivers give you dirty looks! Seriously, it seems this year there are more "lite" trailers and 5th wheels available. Like one poster said, smaller and lighter may be the future of RVs.