This is exactly the symptoms we where having this spring. I got a rebuild kit and rebuilt the head. In my case, the joker valve was quite rotten as well as several other pieces. The rebuild kit did the trick. The hardest part was getting the toilet out without making a huge mess as all the pipes are full of water! Here is what I did, may or may not work for you.....
1. Pump out
2. Flush a LOT of water through the system via the toilet pump to try to clear out as much of the nasty as you can.
3. Pump out again.
4. Here is where I got lucky... I found I could unbolt the toilet and lift it up quite a ways with the outlet hoses still attached. (I think I took off the water inlet hose before this). Holding it up I was able to pump the toilet dry and not get any water back in the bowl, and a lot of the water in the hoses drained back into the tank. Then, while still holding the toilet up (ugh) I took off the other hoses, kept it at an angle and was able to get the whole thing out of the boat without dumping water everywhere.
Whether or not you will be able to get the hoses off without dumping nasty all over will depend a lot on your particular setup. If you can not, perhaps you can get it high enough to fit a small dish pan or cut down bucket or bottom from a milk jug or anything under the hoses to catch the nastiness when the hoses are pulled off.
No matter what, this is a less then desirable job, but I found it turned out to not be as bad as I feared all the way around.
Good luck, and be sure you have lots of soap, water, and paper towels on hand! A few helping hands made a big difference for me as well.
1964 Islander 32
Saint Helens, OR