Did you get your toilet fixed?
About two years ago, I got one of the newer Dometic toilets where you can change the seal by reaching inside. Another advantage is that you can use a toothbrush to get rid of lime scale around the seal. I also was able to scrape the remaining lime away with a small screwdriver because you can slide the seal around slightly. Lots easier than taking toilet apart, and now that I scraped the seal clean, it holds water again.
That alone saved me money and bother of having someone replace the seal. Getting too old to do myself because of the weird positions you have to get into.
![Mad :x](./images/smilies/icon_mad.gif)
So it might be worth it to you get a new toilet with this kind of seal in spite of slightly extra cost.