Does anyone have experience with holding tank contents slowly backing up into the head?
Unless the holding tank is so full it's overflowing, there's no way that tank contents can back up into the toilet because the head discharge line is connected to a fitting at the TOP of the tank, and last I heard, sewage can't jump. So the bowl contents aren't getting to the tank.
Three possible causes:
1. a blocked tank vent. When air in the tank displaced by incoming waste can't escape out the vent, the tank becomes pressurized, creating backpressure that won't allow flushes to get to the tank. If a blocked vent prevents air from replacing tank contents as they're pumped out, the pump or overboard discharge pummp will pull a vacuum that won't allow more than about a gallon to be pulled out...in which case, your tank just might BE full to overflowing even though you think it's been pumped out. If you've noticed that a manual toilet is getting harder to pump, or an electric toilet has started "burping" and spitting up after each flush, that's almost a guarantee that your tank vent is blocked. A new joker valve won't help.
The most common location for a vent blockage are the vent thru-hull and the vent line connection to the tank, both that end of the hose and the fitting on the tank.
The cure: Clean out the vent thru hull and the vent line connection to the tank.
2. The toilet discharge hose runs up and over a loop (not necessarily a VENTED loop), or it's just an uphill run from the toilet to the tank inlet fitting...and since water runs downhill, that's what it's doing...because you aren't flushing in the dry mode long enough to push the flush over the top of the loop or get the flush to the tank. A new joker valve may prevent the back flow from making it into the bowl, but only for a short time...until enough flushes stretches the slit open. However, joker valves in manual toilets should be replaced annually, or at least every two years, because they're essential to the efficient operation of the toilet. Find the article titlled "Joker Valves 101" in the Sanitation and Plumbing forum archives for the explanation of what a joker valve REALLY does.
The cure: learn to flush longer in the dry mode. Few people realize that any manual toilet that's working anywhere NEAR factory spec can move bowl contents up to 6 linear or 4 vertical feet in the dry mode. Learning how to use the dry mode to do more than just push the flush out of sight can increase the number of flushes your tank can hold by up to 50%.
3. A blockage in the head discharge hose...either sea water mineral buildup in it has reduced the diameter enough to all but block it, or somebody flushed something they shouldn't have.
4 A combination of any or all of the above.