I run with a full fresh water tank pretty often, as I camp in places where the fresh water may not be to my wifes taste. I don't seem to have to drive nearly as far to dump the tanks though.
Still, I know I've put a few hours on the road with at least half-full tanks.
2012 F150 SuperCrew EcoBoost 4x4 XLT/XTR 145" WB 3.31 Axle - GONE
Five other ford trucks - all gone
Now rolling in a 2010 EconoBox Recession Fighter