Since it's a totally separate area and fitting, anybody winterizing would have had to blow the flush line separately. It's very possible that they missed it, I've done it myself.
That said, most of the time the black flush check valve is accessible. Mine is under the bathroom sink, way up high. Follow the natural path from the hoes fitting to the cabinetry inside, I bet you'll find it, and your problem. They are known to be troublesome. Mine came apart.
I haven't done it on this camper, but my last one I replaced with a brass check valve.
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Doug and Patience and Shelby the mutt
2015 Crossroads Sunset Trail Reserve ST26RB
2013 Ford F-150 3.5 EcoBoost XLT Supercab 4x4
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