Sounds like the pump is cavitating due to the suction tube not going all the way to the bottom of the tank. I would either install a longer piece of suction tubing ensuring it goes all the way to the bottom of the tank OR cut a tee into the tank low point drain and draw off that. My fresh water tank has a hose barb at the low point with a 1/2" tygon tube running to one of those cheap white plastic spigots mounted to the frame rail for draining the tank. Tee into that.
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.