It depends on the speed of the truck and the height of the camper above the cab of the truck. I get 10-12 MPG with pricy diesel fuel and the 10 MPG is when there is a 15 MPH headwind.
Fuel is the least expensive part of owning a camper and you should think in terms of the actual difference between driving 3000 miles each year and paying for $5 a gallon and the cost at 15 MPG with 200 gallons of fuel or 10 MPG and having to buy an extra 100 gallons of fuel. $500 a year difference is not that much and also no reason to pay the $10,000 premium for a diesel truck.
Go with a gas powered 2500/3500 truck and 3.73 or 4.10 gears and you will get the best overall combination of performance and economy.
With a 1/2 ton truck a popup camper is an option as they will weigh a great deal less and as they stick up less there is a lot less air drag to overcome.