I called Dometic last summer for a replacement. We had an odor which from all accounts and previous posts said to contact Dometic because the toilet had a sealing issue. When I did (you need to have the serial # which is down near the foot lever), they told me that my serial number was after the modifications were already put in place at the factory and I do have the new modified one. Nevertheless, they sent me a new 300 anyway (early on in the program, before they found a fix, they were sending out 310s as replacements). I never installed it because it turned out that my problem was that the black tank shifted during travel and the minimal bracing used to secure the tanks had allowed the black tank to slip out of it's cradle (so to speak) and the pipe joints into the tank had lost their seal. That's where the odor was coming from. Dealer took care of it right away under warranty. Resecured the tanks with Stainless strapping. No more odor!
So, just a caution, odors with the 300 aren't necessarily from the toilet. There have been other posts about the tank slipping issue. With the underbelly covered, it's hard to know for sure unless you dig into it.
Mark, Vicki, & Scout THE dog
2015 Hemisphere 282RK
2010 F250 Super Duty
1992 Goldwing Aspencade