When we were fulltimers, our 5.9 ISB gave us between 9 and 11 mpg, touching 12 mpg a few times. That's a very good engine and we thought our 38' HR Endeavor/Freightliner was quite peppy, compared to previous Airstream/Chevy/Isuzu and Fleetwood/Oshkosh/GM motorhomes.
I use my calculated mpg to improve my driving habits. For instance, I've found that if I reduce my mph from 65 to 60, I typically will increase my mpg by .5 mpg. Over a long period of time, that's a lot of $$ that I've saved. In addition to increasing my safety factor, I also increase the mile I can get out of my tires. The engine will run a couple of degrees cooler and I have less road/wind noise. I use to accelerate on on-ramps at WOT... I now (when traffic permits) only use about 1/2 throttle. This too has increased my mpg...minutely, but never the less, it has increased. I climb grades in the highest gear possible that the engine/trans will allow.
These are just a few things that I've found will increase my fuel mileage and also promotes safer driving. The increased mpg is kinda like the icing on top of the cake.....just makes it all the better and something to look forward to.
Ron & Sandie
2013 Tiffin Phaeton 42LH Cummins ILS 400hp
Toad: 2011 GMC Terrain SLT2
Tow Bar: Sterling AT
Toad Brakes: Unified U.S. Gear
TPMS: Pressure Pro
Member of: GS, FMCA, Allegro
One more thing about the OP's one tank mileage. As I mentioned above, I have exactly the same MH, same year, same engine. Because of the fill position on this MH the amount of fuel you put into the tank can/is affected by the way the MH is sitting. A little out of level, left to right or front to back makes a big difference in the amount of fuel you can top off. I've gotten some freakish mileage figures (both good and bad) that I can attribute to the angle the MH was sitting when previously filled and last filled.
Never the less, that year, engine, MH will get extremely good mileage compared to comparable MH's.
hershey - albuquerque, nm Someday Finally Got Here
My wife does all the driving - I just get to hold the steering wheel.
Superman was an illegal alien.
Expedition - Suzuki Grand Viagra
Reminds me of when I was young and tricky, I used to go put in a couple of gallons of fuel into my neighbors new VW and then go back a week later and siphon out a couple of gallons. It really had him confused. Almost as much fun as when another neighbor was adjusting the tension on his new automatic garage door opener and I used his wife's remote to really confuse him. After about an hour, my wife made me stop.
Enjoy your numbers. If you don't want to know the real mileage, just don't ever check it again.
I don't know much about your rig, but on my gasser, when the fuel pump kicks off I can easily fit in 5 gallons, and sometimes more. If your tank was originally filled "to the brim" and then when you refilled it, it wasn't, a 5 gallon difference in the fuel used brings your MPG down to around 10.5 mpg.
As others have said, far too many variables to determine in a single fill-up. You need more data. For what it's worth, I seem to get my highest MPG numbers on the shorter trips, but the longer trips always seem to bring me back to reality.
Enjoy the numbers, just don't be too disappointed if those don't hold true long term.
2005 Georgie Boy 3625 DS on a Workhorse W-22 (Class A)
Rick, Gail, 1 girl (17-Angel, Lexi96.org), 1 girl (12), 2 boys (13 & 10).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.