Just got back from a camping trip and our rear bathroom toilet is now leaking water back into the tank. The bowl will not hold water. I disassembled it and cleaned everything. Reassembled everything and it still will not hold water. I contacted Dometic Customer Service (800-321-9886) and was told the seal & ball valve need to be replaced. The original part number for the seal alone (385310677) is no longer good. I believe they've had a problem and now have a better replacement seal but it requires a slightly different ball valve to mate to it. They are not replacing anything under warranty. The cost for both is over $80 plus shipping.
It really stinks, no pun intended, that a product that gets regular use only during camping trips would wear out so quickly. I believe Dometic found this out and now have a better replacement seal but won't own up to the problem of the faulty seals they originally used.