I had almost the same problem. Sea water came in and looped to the height I wanted water to stay in the bowl. Sewage from the pump more or less dropped to the holding tank. I noticed that my holding tank would get full in about two days even without use of the toilet. The system was siphoning. Once I put a vacuum breaker in the sewage line wallah everything has worked fine for several years now. I can see very easily that you could have the same problem the other way and siphon all the water out of the bowl back into the sea. Install a vacuum breaker.
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