I kept gas records for 8 years on my 28 foot Class C. The tank was 55 gallons.
My generator draws out of the same tank so that throws off your MPG.
My annual average was always hovering around 8 mpg with 55 gallon tank and I was fully loaded.
First off I never let my gas get below a half tank except in emergencies (running from hurricane!)
I wanted that half tank cushion for emergency generator use. When I had a breakdown in 99F weather on side of hot steamy interstate, I so needed that generator to stay alive and cool until help arrived which took several hours in spite of me being on the outskirts of a major city. I was towed to a closed repair facility. The humidity was out of sight, there was no electric hookup since they parked me outside of the locked gates. I ran the generator another 12 hours just to stay cool until they opened up to make repairs to my rig.
So you're probably going to be looking at buying half tanks of gas at a time, if you want the security of being comfortable if things go wrong. I always topped up my gas before setting up camp, that way if anything went wrong, flood or hurricane... I had a full tank of gas to hit the road.
Most of the time I traveled with a full tank of water too. I was fulltiming 24/7. While I did not tow a car, I had a bicycle and a cargo carrier hanging off my hitch.
I ran my generator 1-2 hours, at least once a month to keep it happy. Other times, especially in summer, I used it more because I traveled with a dog. When I was stopping to shop, I had to run the generator to keep him alive. Mostly I could drive with just the dash air running. This will work fine in a Class C but not a Class A.
However, there were times when I traveled on hot steamy days when I did run the generator and AC because I wanted the rig pre-cooled when I arrived at camp.
My driving style is rather sedate, I was slow to accelerate and often coasted to my stops. I didn't really push it hard and often chose by ways and scenic routes where I could putter around at 45-55mph rather than breakneck speed of 80 on interstates. Even on the interstate, I preferred 55-65.
Sure there were times when my average MPG would zip up to 10mpg, mostly in the winter cooler months and flat lands of Florida, but then the summer and extra generator use would bring it back down again.
When planning my trips, I always figured gas costs at 6pmg and added another 50% to the cost for detours and whimsical sightseeing. This way I had leftover gas money after every trip. 8)
I hope this helps!
