I'm very happy with how Dometic has resolved this issue for me!
I have a 2014 Shamrock 23ikss. After only the first night of camping, my wife and I noticed the foul odor from the bathroom. A quick search online brought me to this forum thread. I contacted Dometic on a Monday. On Thursday, I had a new series 300 toilet at my doorstep, apparently they fixed the seal so that the problem shouldn't occur again. I carefully removed my original 300 toilet from my camper. Outside, from the rear of the unit, I could see liquid sewage that had collected on the inside. Then as I unboxed the new 300, I was very disappointed to find that it was damaged. The two plastic parts at the top of the bowl of toilet were split. It appears they are glued and snapped together and one of the two screws in the rear of the unit had been stripped.
I immediately contacted Dometic on the phone (Friday morning) . They were very understanding and agreed to send me another new unit, this time a series 310.
When I saw QuietTimes post above regarding the size difference between the two models I became concerned that the 310 may not work for me. It just arrived this afternoon (Tuesday) and.... it fits! I am very pleased with the fit finish and flush of the series 310. Thank you Dometic.
To reiterate - the 310 fits fine in a 23ikss.