It is awfully hard to figure milage. Even the best math. expert cannot figure on all the conditions that affect the MH every mile going down the road. Each coach gets different milage, some older rigs better than some newer units and the reverse could be true. But wind, weather, traffic, road conditions, mountains, elevation, running the generator for AC,etc. can have an impact too. Your mpg range could be anywhere from 6 to 9. Ask the dealer or manufacturer what the average mpg is for the model you are looking at and figure it will get worse in all the conditions listed.
For our coach towing our mpg is usually somewhere between 8 & 9 (see our siggy). We usually end up hitting all the conditions that ruin mpg. Running the genny uses maybe 1/2 gal. per hr and towing makes about 1/2 to 1 mpg difference.
If mpg is a deal breaker...well I don't know what to say...owning, maintaining, driving, and RV park fees is not cheap by any means. We have found it a wiser choice than traveling via air or car and staying in motels. Considering the cost of those items it pretty much an even deal or even better deal.
My AVERAGE MPG for last year towing my trailer was 7.7MPG - that was my 39' rig at about 26K LBS and towing a 10K LB 30' trailer with a 1966 GTO, golf cart and the kids toys. My speed was usually never under 65MPH - typically I was in the 70-75 range, with some time above that and some below. It seems to hit another sweet spot for power and MPG when I'm above 72MPH - I know about wind drag, but the coach really glides nicely.
When I made a trip in August - MN - NM - UT - MN I was towing a 09 VW Jetta wagon TDI - that trip was a bit better. If I kept the speed to 61-63MPH then I would pretty consistently be 10-10.5MPG (don't believe it? it's true! I've been tracking with my silverleaf AND hand calc to calibrate the silverleaf. I have had the speedometer calibrated to my bigger tires and the silverleaf. hand calc now always jives.)
So my sweet spot is right @ 62mph.
What have I done to help my MPG's?
Synthetic fluids = engine, trans, differential
Michelin Super Single rear tires (yup the 14" wide suckers)
Removed the satellite dish from the roof (less wind drag)
Chopped off the stock muffler and put a straight thru 5" semi truck muffler (donaldson - not that awful Aero 5050XL - I have a lightly used Aero 5050XL if anyone wants - too much drone for me)
My trailer is smaller profile than the coach, so it "fits" into the air hole that the coach punches...
I'm pretty pleased overall with my CAT 3126/Allison 3000 combo
2009 Newmar AllStar 4154
2014 VW Passat TDI - toad!
Featherlite 28' Enclosed car hauler
1966 GTO - super cool car as seen on Driven1
lucky to get 6mpg with a 33 ft Winnebago, towing a 10 Ford Escape on a dolly..appr 21000 total weight...wish it was better--but i look at it this way...the Ford Escape is getting 1000 MPG whilst it is being towed
31,000lb.coach with 06 Saturn VUE. 330 Cummins with 39500 miles.Recently had it at a Cummins dealer.Laptop says 8.1 mpg over the lifetime of the engine.I try to stay at 63 mph & only use the economy mode occassionally.
F53 33 foot no slides 460 towing a Saturn SC2. All stock
Never keep track of MPG till this spring and then only once. Given a 25+ mph tail wind, doing 2400 rpm's (about 62 mph) I got 10 MPG.
I use the toad to explore, and the 460 to get me there and house us. To fill up from 1/4 tank is $200. After a full day of driving I'm back at 1/4 tank. The real question for me is how many days can I drive back to back without camping for awhile.