I am buying a new tow vehicle. I am getting either an F350 or F450, I am sticking with DWR, My boat currently is only a 32 Advantage, but planning on upgrading to 39' (make to be determined), I also pull a 40' fifth wheel toy hauler on occasion. How much different is the milage between the two? I will be driveing it empty more than loaded. But when loaded I need the capacity.