As Dan mentioned, the black tank flush does not need the use of a pump to operate the flush. Water pressure from the supply hose is all that is necessary.
It would seem odd that a water pump switch would be located on the outside of the trailer, especially near ports that create water splatter when hooking/unhooking.
Most times, the water pump switch is located inside on the control panel.
Did you purchase new or used?
Maybe it was something added by someone else?
Could you post a photo?
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