We have a pre-rinse valve that allows the bowl to fill before use. When flushing liquids, we obviously have no issue. When disposing of larger solids is when the clogs typically occur. We have tried pre-flushing to wet the sloped pipe, but nothing works very well. We have to use the 1/2 bath in the back of the coach when needing to deposit solids (because it has a straight vertical connection to its own black water tank and doesn't clog).
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