There is an anti-siphon valve in the flush line behind the shower wall. It has failed, not the first time this has happened. There are posts of how to cut the wall in the pantry to access the valve. The other solution is the find the lines behind the wall in the pass through compartment, cut them, and rejoin them with shark bite (or similar) fittings to bypass the failed valve. A Search will uncover a number of posts on both solutions.
Carl (n Susan) Youngren
There is more to life than fuel mileage.
2012 Montana 3700RL Big Sky Package towed by a 2015 Ford F350 6.7L PSD 4WD CC LWB