I had a 1999 f250 short bed with a 1997 Lance squirelite 165s i got 14 mpg. I got solid 12 pulling a 2003 wildcat fbs26 with the same truck. You can look up the specs of the 1997 lance campers to compare with what ever you get for a truck camper. That same 1999 f 250 got 12mpg pulling a car trailer with a ranger pick up on it across the western US
My guess is that your mileage will be very close to 12 to 13 with a bigger camper. The 165 is about the smallest lance camper they make, at least in 1997.
I have a 2002 Lance 1130 with a '06 f 350 drw now and I dont even check the mileage, it is what it is. I hate to say it but with fuel $7.00 plus it is starting to make sence to drive the honda and stay at a motel, in some situations. I really have to want to be in the camper to spend the extra money.