It has to be the black tank flush. It should have a check valve in it between the tank and where you are. When you winterize, just blow it out with water pressure in case there is a loop that traps water.
You can verify if that is what it is by putting water to it and see if it goes in the black water tank.
2011 351RLTS Eagle, Mor/Ryde suspension & pin box
06 F350 Lariat PSD, SRW, LB, CC, EGR delete kit, 16K DrawTite hitch, Timbrens, TST TPMS
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