Larry,
This is a significant inconvenience, if not a significant problem. There are many posts in the forums about this problem. I'd encourage you to do a couple searches and read what's already there.
The problem, in my opinion, is a design issue. Now, I'm not a potty engineer by any stretch, but I've gone head-to-head (Pun Intended) with it a few times.
In just shy of two years full-time in or BH, I have disassembled, cleaned, and lubricated the seals a couple times; replaced seals twice (warranty), and replaced with a complete new toilet (warranty).
Your next task should be to contact Sealand (Dometic) describe your problem and request a seal/rebuild kit. You, like some of us will likely get it gratis.
Good Luck.