The hose from our toilet does not go to the top of the tank. They go to the bottom of the tank...
That has to be what's causing your problem...waste MUST enter the tank at the top, be discharged from a fitting at or on the bottom. Some tanks
may have a discharge fitting on the top of the tank, but it will always have a pickup tube inside the tank to the bottom. To answer your question, A joker valve in the tank connection is NOT the answer...water and waste weigh 8.33 lbs/gallon. To use just 40 gallons as an example (with four toilets, I suspect yours is much larger), there'd be 333 lbs of pressure trying to turn a joker valve inside out...and it will succeed! I'll leave the rest to your imagination.
The ONLY solution to your problem: re-plumb the toilet discharge to enter the tank at the top. If the other toilets on your boat are also connected to the tank at the bottom, they're a disaster waiting for just the right moment...in which case the whole system needs to be replumbed.
Re-plumbing one toilet to a tank is a relatively simple job that any competent boat yard will know how to do correctly....however, 4 toilets connected to one tank on a 46' boat can be a real challenge that whoever created this mess obviously wasn't up to. There's a company called "Marine Sanitation" in Seattle
that will know how to do it right. I strongly urge you give them a call. 206-633-1110 They service
the whole PNW, and can come to you if you don't want to bring the boat down to Seattle