I'm thinking of putting a Past Time camper on my truck - a basic model - that weighs about 1500 lbs with fuel and water batteries - no personal gear..... here's my ????? - the load is light but the air resistance is there - does anybody have an idea of how much a fuel hit I'd take with this setup - currently I can get 19-20 on the road... . and 15-16 around town with my lead foot... ... I also have an above cab (8 -10 inches) pickup shell... ...