Here's my view on this. Caveat: based on experience in the Northeast - Buzzards bay, Rhode Island Sound and environs.
Unless the joker valve or other rubber components are really junk, they should last quite a long time.
The key to keeping a marine head working, trouble free, is to flush it thoroughly. Lots of water. Salt water works fine.
A cup of vinegar flushed into the head and left to sit for a minute or so - is a waste of vinegar. It will do exactly nothing. If you have salt or calcium deposits clogging the hoses, you are probably not flushing with enough water. If you doubt this, take the hose off and scrape some of the deposit out and out it in a glass with vinegar. After 60 seconds at room temperature, how much of the salt and calcium deposits have dissolved? Nearly none. If you wanted to dissolve these deposits in the head parts and hose, you'd have to fill it and leave it for quite some time. The vinegar might have a bad effect on the head parts, depending in what they are made of. Some rubber-like compounds not so bad, some bad.
Likewise, the final flush of fresh water is a waste of fresh water. You would have to fill the passages with fresh and leave it for a long, long time to have any impact on salt or calcium deposits.
Backups can be caused by failed joker valves. Failures can be caused by a breakdown of the material of the valve; by an object or deposit preventing it closing fully; or too much back pressure, as in a pressurize holding tank, due to a clogged air vent, or a too-full holding tank. The first is unfortunate. Apparently the W-C parts are pretty good. The second - flush with lots of water. Might have to disassemble to clear in an extreme case. The last, clear the vent or pump it out. Ours will back up a bit when the tank is nearly full, say to 17 of the 18 gallon capacity.
In my experience I've never had to take the thing apart. I've always been abel to fix backups by flushing more water (to clear an obstruction holding the valve open) or pumping out the holding tank (because it was nearly full). Once we had a clogged vent and the plastic tank was pressurizing when we flushed, and the pressure was causing a backup. Clearing the vent fixed it.
Now, it could well be that in different places, with different water and temperatures you would end up with chronic salt and calcium deposit problems. But, in that case, a cup of vinegar for one minute once a week will not cure it, in my estimation.
As far as pumping in dry mode moving waste through the plumbing - I don't think so. If it works, I'd like to hear the explanation.
(My salt deposit dissolving experience is based on real experiments I have performed while evaluating methods of cleaning and maintaining SCUBA equipment, and evaluating the effectiveness of commercial salt dissolving products, like Salt-X. Acetic acid works, but at room temperature at vinegar dilutions it will do nothing significant).
