Unless the holding tank is full to overflowing...
There's no way it can back up into the toilet...the tank inlet fitting is at the top of the tank...and since water can't jump, the tank has to overflow for it to get into the head discharge hose. So if it IS waste water, there are only two possibilities: 1. your tank has to be full. If you're absolutely certain it wasn't when you left the boat, the most likely source is the tank overboard "dump" through-hull. 2. It's not from the tank, but is waste/flush water left in the head discharge hose in an uphill run to the tank running back downhill into the toilet. Depending on the length of the uphill hose run, that could be enough to fill the bowl. There is a third possibility, but you'd have other symptoms...a toilet that's very hard to flush--a LOT of backpressure--and immediate bubbling back after using the head: a blocked tank vent, causing the tank to become so pressurized that nothing being flushed is going into the tank...it's just building up in the head discharge line. I doubt that's your problem, though...'cuz if a tank is THAT pressurized, you'd be asking different questions--maybe after you showered off the unexpected "bath." However it IS quite possible that a blocked tank vent may mean that the tank is full even if you think you pumped it out or dumped it. Have you made certain that the tank is empty (or at least not full)? If it's full, and shouldn't be, you have two problems to solve. The wet/dry valve is in the toilet intake--has nothing to do with toilet discharge...so how could it have anything to do with backflow from the tank or head discharge hose? And if the intake seacock is closed, that would rule out any possibility that what you're seeing in the bowl is intake flush water. Bottom line...the problem isn't in your toilet pump....it's downstream of the toilet. And my guess is, the tank overboard thru-hull is either open or leaking. The joker valve in the head discharge fitting is prob'ly long overdue for replacement, though...joker valves should be replaced every couple of years. If you don't know what a joker valve is, check the drawing for your toilet in my book. A new one should slow down the backflow, but it won't cure the real problem, though. Most of this is covered in my book, btw.
