There are no dumb questions - but there are less than reasoned answers. If there is a place to fill water tank before you arrive, then fill there; otherwise, go with full tank. We are going to older son's place in Sangre de Christos (7600') to west of Las Vega, New Mexico. He uses water catchment for washing and hauls water for drinking. He is off-grid (6000 or so watts of solar) so we fill 81 gallon tank and another six x 6 gallon water containers when we visit. We carry the water containers when we travel so that we can always go somewhere to get fresh water. The Open Range system has a siphon for winterizing that also hauls in (at different valve settings) water into main tank at about 1.5 gallons/minute. It works well.
We are weight police and do weigh the rig on CAT scales with the 81 gallon filled, the 35 gallon fuel tank filled, and the 50 gallon Transfer-Flow filled as well. The trailer axles are rated at 5250 (10500), the dualie rear axle at 8300# and the front axle at 4500#. Last weighing two weeks ago had us at 9500# on trailer axles, 7200# on rear axle of dualie and 4300# on front axle. 81 gallons of water is about 648# and it can make a difference to folks that are pushing the limit (and a lot of folks go way beyond the limit). I know I did go off subject a bit - but there are reasons to not travel 1000 miles with water you do not need. Admittedly, we fill up in US before we go to Yucatan so that we have potable water on the way down; then use the local water for washing and bottled water for drinking. We thoroughly sanatize when we return to USA.
Reed and Elaine Cundiff