Neighbor had it in his 2018 Montana. He replaced the valve under the sink twice. Many chemicals and hours of flushing. Next a hose on the roof nd water on. Nothing worked. No poop pile either.
Easy fix! I saw he had not trashed the Thetford junk toilet. There is a plastic vacuum valve at the top rear. A $35 kit and 10 minutes.
The bad part is he said any more problems he comes to me 1st. If he doe not order a Dometic 320 this week Ill block his #.
Not really but I told him so.