I recently purchased a 07 Classic 4x4 stock drive train and 285 tires. I get 14+- in the city and 16ish unloaded on the hywy. my camper is 8k and i get 9 to 10 best I recently pulled a boat (9-10k) flat land and got 6mpg is that to be expected? should i be looking for somthing wrong? my speed was approx 68 and 1600 rpms. i had the GM place look at it and they say it is in spec's. she does not struggle at all just poor milage, any thoughts.