Double cab? Doesn't it only have a 5.5' bed? No camper on it!
I have a 2013 Tundra extended cab with 6.5' bed and it gets 15 to 16mpg sans camper and with my Hallmark Milner it gets 11.5 to 12.5 mpg. I just ordered two 3 gal gas cans to extend the limited range of the 26gal tank. But remember gas is not the major cost with a truck, maintenance is. At 90K miles, no costly problems so far.
Bill