So, I currently have a Chevy Silverado 6.0 liter gas truck. 4 full doors, long bed. I tow a 37ft bumper pull rv with it full time, around 9000lbs loaded. Truck bed is full, maybe another 1200lbs. I get 7mpg average, and that’s across the country a few times. Unloaded maybe 10mpg. I run up to about 80mph when towing where possible. My Dodge Ram before it with a 12v Cummins diesel got 14mpg loaded similarly, and about 18-20mpg unloaded (I kept about 500lbs of wood in the bed all the time so it was never really empty). It’s about $130 to fill up my Chevy right now with current gas prices. We had to do it about six times to move up north this summer for work.