Seems to me that it would be a whole lot easier to flush it out when you flush out the tank and the rest of the system in preparation to winterize...'cuz you're gonna have enough to do to get it started again with the right level of salinity without having to mess with
flushing it out 20 times first.
And btw...speaking of
TANKS and winterizing... The tank should be pumped out, then VERY throughly flushed out to remove any sludge in preparation to winterize. It's not hard to do:
After pumpout, put enough water into the tank to cover the bottom to a
depth of 4-6"...it needs to go into the tank via the
deck pumpout 'cuz that sends it into the tank at the bottom to stir up any sludge.
Pump that out. Once is enough after every pumpout, but several times a year--and ESPECIALLY in preparation for extended layup!--that should be repeated until only clean water is being pumped out. Before
layup, there's one more step: put water in the tank again, and this time open the thru-hull and run the macerator to rinse it and the
plumbing connected to it.
If you have a washdown
pump, it's even easier to us it to flush out the tank
offshore: dump the tank...then use the washdown nozzle to put several inches of water into the tank via the
deck pumpout fitting...and restart the macerator. Continue to let the water run till the macerator is only dumping clean water.
To add
antifreeze (ONLY the potable non-toxic "pink stuff," never automotive), simply disconnect the
head intake line from the thru-hull, stick it into the jug of AF and flush the
toilet to pull it through the whole system and push it into the tank.
If your toilet uses onboard pressurized flush water, the toilet is winterized when the fresh water system is winterized...so just pour antifreeze for the tank down the toilet.