Just flush a cupful of undiluted distilled white vinegar all the way through the system once a week...or at least last thing before the boat will sit. Do NOT leave the vinegar sitting in the bowl...it won't do a thing for the head discharge line there. Don't rinse...just make sure you've flushed it all and then go away happy.

And btw...you really do need to use it once a week if you're continuously aboard and/or last thing before the boat will sit EVERY TIME. Just "every couple of months" or "several times a year" is a useless waste of vinegar. If that's all you're doing and you think it's working, you don't need the vinegar.