Black tank flush not flushing

It could be frozen, here's my experience. I have a 2015 178, my check valve was in the backflush line system backwards. It is in the shower wall. Never did work, and I decided not to remove bathroom sink to fix it. I eventually bypassed the installed flush and moved the capability to the drivers side. As far as froze, there's other ways to determinr versus the backflush line.

here is a vent line, however the backflush is a very simple system from a hose nipple to a check valve to the actual flush attachment on the tank. I removed my rear wall access panel which enabled me to see the check valve and verify it was installed backwards. To remove and reinstall correctly would have meant I would have to remove sink to access it, I couldn't get to mine thru the access (too low in the wall). It is located in the rear latrine wall. I decided not to do that. I disconnected at tank and left it in the wall. FYI - Numerous reports of this valve installed backwards around the 2015 timeframe, there was a post on this forum about removing sink to fix the faulty valve.