While I'm taking it out I may as well install a new Raritan PH Super Flush as I am getting some drain back from the holding tank and as Peggy said I need a new pump.
The inlet on the tank is at--or on--the top of the tank and tank contents can't jump. So what's running back to to toilet isn't likely to be from the tank unless it's full to overflowing or it only happens when the boat is heeled hard over on that tack. So what you're seeing is waste in the toilet discharge line that isn't making it INTO the tank. A new toilet won't solve that problem, 'cuz if you've only begun experiencing backflow recently, that's a strong indication of a blocked tank vent.
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing,
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works. If there's a screen in it, knock it out...screens cause more problems than they prevent. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...replace the vent line.
As long as you're doing all this work, I recommend you replace the "vent" thru-hull with an open bulkhead or "mushroom" thru-hull. Not only will this allow a lot more air exchange via the vent, but you can prevent any future vent blockages by sticking a hose nozzle against it to backflush the vent line every time you wash the boat.
You might consider getting a copy of my book (see link in my signature below)....the title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie