The amount of water pressure resulting from a full tank of water, or a half tank of water is still slight in comparrison to the pressure required to flow 2 to 2.5 gallons per minute, which is what the water pump in the trailer is rated at. If the water tank was mounted on the roof, the weight of the water, combined with gravity would allow higher pressure when the tank was full in comparison to when the tank was partially full, but again, not by much.
IMO the only "Energy" concern with water is the energy used to move it, you will use more fuel pulling a trailer fully loaded with water than pulling the same trailer with empty tanks, especially in hilly terrain. The water pump is only drawing power when it is in use, but it uses 7amps @ 12v, which is 84 watts per hour. For a comparison, the average PC uses 78 watts per hour, but the duration of time the water pump is actually on is quite short. The water pump in the Fun Finder will empty a 30 gal water tank in 15 mins, so you will probaly run out of water before you drain the battery!!
2006 Nissan Xterra
B&M Trans Cooler
2011 Nomad 206
Panasonic/Pioneer/Kicker ~ Audio/Video System
Days Camped /2009 / 33