Dometic Sealand Toilet will not shut off -- Overflow

Here is another brand: Flair-It fitting.

The reason I like the Flair-It (and possibly the Watts since it is rated for behind wall installations) is that it squeezes the tube, and does not depend on a single O-ring for installation. The problem with some of the "*-bite" fittings is that only an O-ring separates the water from the floor of your rig, so if it gets nicked on install or if chlorine in the water erodes it, it will start leaking.

Different brand

on Amazon

Looks like

Changing the flush valve can be anywhere from simple to very difficult depending on the specific toilet.

But by all means DO install a shutoff valve in the supply line while you're working on it! I got one at Lowe's that looks a lot like this one, except it's a push-to-connect style like SharkBite. It's good to be able to shout the toilet off and not lose water to the rest of the coach.
About a $10 part.

We replaced our original toilet with an all ceramic Sealand Concerto.

While we were at it, behind the toilet we installed a ball valve in the water supply and a cut off switch in the 12 volt supply.

Fortunately we've never had to use either one, but they are ready in case of need.

If you're making repairs, you might want to consider putting in an emergency shut off. It will allow you to use the rest of your RV water system while waiting for parts, etc.

Easy fix, get the part and its a pretty straight forward replacement. If your in a bind to have it working properly. Remove it and dissassemble the valve, clean the sliding surfaces and apply vasoline as a lubricant and re-assemble. I did that to mine and that was over 9 years ago, fulltiming.

Find parts for YOUR toilet HERE

Mine did the same thing several years ago. CW had the parts in stock. It was very easy to replace.