You might have a bad atmospheric vacuum breaker that goes to the tank flusher. Might be able to temporarily bypass to confirm if it is the problem. Most are located under the lav near the top of the cabinet. They do go bad.
Chris & Maria
2012 36RL, Full Body Paint, Dual Awning, Dual a/c, Splendide Combo, Fisher Paykel dishwasher, 6.5kW Onan, Goodyear G614's. 2015 F550 6.7l, 4X4, Lariat, CC, 176" WB, Long Box, B&W Companion