HI. I had the one of your issues with my 2019 26DBHL, the water not holding in the toilet. I had mine repaired under the 1 year warranty. I was adamant about this repair, because it is a safety issue (water lock against sewer gas). I cleaned the gasket as per manufacturer instructions with no success. My dealer replaced the seal with no success. Finally dealer replaced the whole toilet and this solved the problem. Be careful on this one, because water holding in the toilet is mandatory to keep you safe. Insist that your dealer fix this problem (under warranty) due to it being a re-occurring problem and the safety aspect of a toilet seal problem.
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Ontario RVer
2019 Alpha Wolf 26DBH-L
F 150 5Litre
The Dometic 300 series toilets have had a design and/or assembly problem. The seal between the plastic base and the ceramic upper section leaks. Not sure of current status; several reports on the forum here of Dometic replacing defective toilets.