My only concern would be pin box clearance when turning.
On a goose neck... no problem but the part of the pin box that sticks down on a conventional 5th wheel will likely hit and then damage the tank, the truck or both.
Many folks us an auxiliary tank or bladder to carry extra water.
On units with a winterization port... it is fairly easy to connect a hose to that port and draw from the bladder/tank when the on-board fresh thank gets empty.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel-Goodyear Endurance Tires
2017 Ford F-Series SCREW 4x4 - Factory Puck B&W Companion
TST Tire Monitor w/Repeater - Sinemate 3500w Gen.
Days Camped: '17=30 ☼ '18=51 ☼ '19=58 ☼ '20=83