I thought I'd share pics and some details of a freshwater head conversion which i did on my 1980's Carver 32.
Why the conversion? Especially at the dock, you don't want that nasty water coming into your toilets and stinking and sitting a while. I've converted to use freshwater from the nice clean water tanks (which i keep clean enough to drink from).

Key parts you will need (per toilet):
1. US Solid stainless steel 1/2" normally closed, 12V DC solenoid valve, $43
2. Watts 288A Anti-Siphon Vacuum Breaker 1/2" brass, $40
3. Momentary push button, 12V, waterproof w/ LED, $4.50
4. A few inches of Pex, some polyetheline braided hose, clamps, marine-grade wires, electrical connectors
5. Sharkbite Tee (3-way) fitting 1/2", $14
6. 1/2" MNTP threaded / push Sharkbite fitting, $9
7. 2 x plastic 1/2" MNTP threaded / 3/4" barbed fittings, $6
8. plastic 1/2" MNTP threaded / barbed fitting (either 3/4" or 1" depending on size of hose going to toilet), $3

Labor: 4 hours each toilet

Key steps:
1. SHUT OFF the raw water intake going to the toilets
2. If you have a raw water washdown, you will need to redirect the plumbing from the raw water intake to just go directly to the pump.
3. Connect the new momentary switch and solenoid to the power wires (+/-) used by the toilet flush. For the forward head this is relatively easy. For the aft head, the black wire ends in in the tall closet behind the portside bed. You have to run another black wire from the closet to the aft sink cabinet, and use a 3 way connector or crimp to connect that wire to the black wire going to the back of the toilet (for the pump motor) and the black wire coming from the circut breaker.
4. Connect the sink cold water supply to the solenoid BEFORE the siphon breaker
5. Tie up the siphon breaker as high as you can get it underneath the sink cabinet
6. Turn the toilet dial to DRY FLUSH and remove the dial
7. Disconnect the old hose from the toilet pump to the toilet bowl

How it Works:
You will have a nice blue LED night-light button that, when you press it, puts fresh water in the bowl from either your tanks or shore water. It is controlled by the same circuit breaker as the flush button. So I'll let you figure it out from there, but essentially you would flush until waste is gone, add some water while flushing to push it through, then add just water to put some in bowl and create a vapor barrier.

Hardest parts:
1. Pulling out the old hose from the raw water intake to the aft toilet. You need to remove the top of the portside bed and get in there and cut up the old hose to get it out of the cable ties. I suggest you just LEAVE IT ALONE and don't waste your time as I did taking it out. Cut the ends to get it out of sight and forget it. That was a bit OCD of me....

2. Getting the threaded plumbing fittings to not leak. Use teflon tape AND potable water system putty.
3. Working in small spaces under sink