When I sail in the slightest swell the bowl fills up (i.e. the waste comes back from the tank) with 'gunk' from the holding tank. I have to keep flushing every hour or so to keep the waste out.
It is possible for tank contents to run back into a toilet discharge line at the top when the boat is heeled hard enough over to let them flow into the inlet fitting. But you didn't mention heeling. Otherwise, the only two possibilities are, 1. the tank would have to be full to overflowing for the material to be coming from the tank, because unless your system is plumbed incorrectly, the inlet on the tank is at the top and tank contents can't jump....or 2. The most likely. Your tank vent is blocked. Air displaced by incoming flushes can't escape, a pumpout or overboard
discharge pump will pull a vacuum that won't allow much if anything to be pulled out... so the system has become so pressurized that flushes aren't even getting to the tank...the backpressure is sending 'em back to the toilet. You THINK the tank is empty or has very little in it because the pump isn't pulling anything out.
First thing to do is check the tank vent...start by opening the pumpout fitting cap to relieve any pressure, and I recommend that you do this at arms length UPwind of it and have a hose at the ready! If the boat has sat for a day or longer, it is possible that enough air along with backed up sewage has escaped through the toilet to relieve the pressure until you start using the toilet again. So don't be fooled into thinking the vent isn't problem...I'd be real money
that it is. The two most common locations 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. Start at the vent thru-hull...use a screwdriver blade, ice pick-whatever works-to dig out whatever is in it. If there's screen
on it, knock it out...screens cause more problems than they prevent. Next, remove the vent line from the tank (easier if you warm it a bit first...you're likely to find the end of the hose and the tank fitting packed. Scrape both out...reconnect the vent line. Now you should be able to pump or dump the tank completely.
Next: replace the vent thru-hull with an open bulkhead thru-hull. All "vent" thru-hulls are actually designed for use on fuel tank
vent lines, so the have spark arresters and the smallest opening possible to keep sea water
out of the fuel
supply. Boat builders use 'em on ALL vents 'cuz it's less expensive than the right one for each application, and they're ok for water
as well as fuel
, but totally wrong for waste vents. A plain ol' open thru-hull will let you stick a hose nozzle up against it and backflush the line to prevent it from ever becoming blocked again. Any waste or critters in the line will be washed into the tank.
Finally...you are gonna have to do a little toilet maintenance
. Replace the joker valve in the toilet discharge fitting...I would'nt be surprised if you find it turned inside out when you remove the discharge fitting. After you've done that, disconnect the head intake hose from the thru-hull (prob'ly be a good idea to close the seacock first) and stick it in a bucket of water to which you've added a quart of distilled white vinegar...flush the whole bucketful through the toilet to clean it out.
And despite your not wanting to take your head system apart, when it comes to systems--especially sanitation systems--preventive maintenance
calls for it. And not only is prevention always cheaper and usually a LOT easier than cure, you get to do it on YOUR terms, at YOUR convenience...which you're finding out is never true of repair.
Although I'm 99% certain the above is the problem and the solution, I am occasionally wrong. If this is one of those times, send me an email
and we'll spend some one-on-one time getting to the bottom of it.