It doesn't matter whether the clean water rinse after vinegar is salt or fresh, either will rinse out the vinegar after it's had time to do its job...although using a couple of beer cups of fresh from the sink wouldn't be a bad idea.
2 x 6 months = a year, which is about the recommended joker replacement interval in other manual toilets, so maybe you DON'T have anything to complain about.
But to be sure, I suggest you check into this forum's sister site cruisersforum.com 'cuz there are a lot of sailboat owners there and Lavacs are a LOT more popular with sailors than with powerboaters. However there are few trawler forum members who also hang out there..."Ranger" is one of 'em.
This is a direct link to the plumbing forum: Cruisersforum.com