Just one question. Is your fresh water tank higher than the low point drain? Typically the tank is mounted to the underside of the travel trailer floor between the frame rails. The "low point drain" is the lowest point in the pressure supply lines not necessarily the lowest point in the entire water system.
Locate the freshwater tank under the trailer, look for a hose barb protruding from one side of the tank right at the bottom. It typically sticks out horizontally. Usually there is a piece of low pressure hose with nylon reinforcement leading to a white spigot type valve mounted to the frame rail.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.