A separate flush water tank is one solution, but USCG regs require that it be completely separate from the boat's fresh water system--no common plumbing
, not even the vent or fill. So while it's ok to be able to switch between fresh from the tank and sea water, being able to auto-fill from the boat's fresh water tank
If you want to flush using fresh water, rerouting the intake line to tee into the head
sink drain line is a better idea. Tee needs to be below waterline to allow flushing
with sea water. To rinse the sea water out of the entire system, close the seacock and fill the sink with clean fresh water...flush he toilet. Because the seacock is closed the toilet will pull the water out of the sink...rinsing out the intake line, pump, channel in the rim of the bowl AND the discharge line. Works quite well with "regular" marine
toilets, manual and electric...I see no reason why it wouldn't work
just as well with a Lavac.