Dennis,
Our 3400RL '07 just started having problems with the toilet holding water also, we got ours this March. Ours is the Dometic Sealand Model 210.
I eliminated the dealer and called Customer Service Number I found in our Manual for the Toilet. 1-800-321-9886. The lady was Very nice. This has been a big and on going problem for them. She said they have a better seal now. (I'm thinking postive on this one)
She sent me seal kit free of charge and arrived yesterday. My husband has replaced them in our Other campers in the past, but this is the first for our Monty..
One thing to remember. It is VERY important to torque it down correctly.
Also a word of caution. The Customer service lady also told me NEVER apply vaseline to the seals. Good luck
Ditto on what the others said about this forum.. It is indeed the Greatest!!! I have learned so much from all the wonderful people.
Sharon