12Gals isn't much weight,....maybe 100lbs if that. I prefer to pull my boat (a 901 with a 53gal tank)
with as little gas as possible in it, just because of the extra weight,...i fill it when i get to my destination. 12 Gal won't make a difference though.....might as well fill it wherever the gas is the cheapest! LOL
Mac
BTW, I pulled mine home from NY and it was almost empty,...I might have 10gals in there......and it didn't make any difference in my truck's mileage,.....i had to stop just as often both ways....