As some water tanks are forward of the axle
it might make some sense to carry a minimal amount of water while towing. The Hunter Compact trailers are laid out in that manner. A full water tank adds about 40 lbs to tongue weight
on our Compact-II.
We carry water in several manners, depending on the destination and trip length:
a) Frozen. We load the refrigerator
with frozen bottles of drinking water before leaving. This helps get the ref. real cold, real fast and provides cold drinking & cooking water as they thaw out.
b) 1 Gallon Bottles: These are recycled spring water containers, filled at home with tap water. We carry up to 6 of these in the the TV and they are usually designated as cooking water.
c) 5 Gallon bulk. We have two collapsible, 5 gallon, Coleman water cubes that can be filled and carried, and are usually used for filling or topping off the water holding tank for the sink, loo and washing water.
d) On board tank: We usually only put a couple of gallons in here with the anticipation of filling when we reach out site. In camp grounds, where it's not possible to fill the water tank directly we use the 5 gallon cubes to fill the tank and then refill the cubes from local water supplies.
If we are going to places like Joshua Tree National Park, a desert park where water is only available at the entrance stations, we fill and/or top off everything at the entrance station or the last known water source and carry as much extra water as practical. Ya gotta plan ahead...
FWIW: We only drink and cook with rebottled tap water. The on-board tank water is used only for washing and the loo.