In my experience, it might just take months for a vinegar soak to actually do any good. For those of us who live on our boats 12 months a year, a month long soak is just not a practical solution. Most boats that get taken out of the water for months, do so because of cold weather. Leaving vinegar in lines in freezing temperatures is not a good idea.
All the other ideas you suggest are certainly good ones, but again, if you are in a place (like all of the waters of the USA) where discharge of raw sewage is never allowed, it is impractical to flush with enough water to clear the lines every time. The tank just fills too fast.
Hollywood, Florida, USA