We wait until we are offshore
and simply flush it over and over again, using the macerator to pump it overboard
each time. It's best when it is a little bit sporty to keep it agitated. If not offshore
then we do it at pumpout.
It takes hundreds of pumps to fill the tank with our Raritan
PH-II manual head but we have an electric raw water deck
wash system so we can fill through one or both of the 1" vent hoses which terminate in mushroom through-hull fittings high up. Those cheesy standard screened 1/2" vent fittings are too small and you can't backflush like this with them. Not only does this fill the tank quickly, but it also back-flushes the vents and ensures they are clean and clear of any obstructions.
The only thing we use for cleaning is Raritan
CP "Cleans Potties." CP and KO are the only things we ever put into our tank other than sea water and what comes naturally from our body holes. We don't ever use fresh water from the pumpout to flush the tank. If we need to flush with water at pumpout we use our deck
wash pump and natural sea water. City water has clorine in it which hurts the good bacteria in your tank. If we want to agitate we can always rock the boat. I have heard ice cubes help in heavy seas. They help scrape the sludge, then melt harmlessly. But again, tap water...
I am sure the Head Mistress will show up soon to add anything that I have missed.