Ye gods and little fishes!!
The freshwater input is separate from the output to the holding tank. Your toilet at home won't contaminate your freshwater system, a boat system is really no different.
That's not true. 1. The water in the fresh water plumbing in your house is always pressurized. Water onboard only remains pressurized as long as the pump remains on. Pressurization prevents backflow and bacteria migration.
2. The fresh water line in your house is connected to a flush water tank that is above the bowl. The bowl is fed by gravity...no contact with the fresh water plumbing. Y'all want to connect the line to the back of the bowl or the toilet pump that has a line that goes directly to the bowl.
3. When the boat's water pump is off, the fresh water system isn't pressurized, leaving bacteria from bowl--and from READYTOGO's holding tank vent line are free to migrate in any direction into your fresh water plumbing. (What were you THINKING??? Bacteria laden gasses from the tank are bad enough....Did it ever occur to you that a holding tank overflow out the vent would put sewage into the head intake line...That's connected to your your potable water supply???? You're playing Russian roulette with your own health and that of every passenger or crew that's ever onboard...and so far you've been lucky)
There's a bit more to toilets designed to use pressurized fresh water than just a check valve...you'd need a back flow preventer (check valve) siphon check valve...if the toilet is electric, a solenoid valve...
There's a much easier way that's SAFE: replace the toilet with one designed to use pressurized fresh water...and there's one for just about every budget:
If you want to stay with a manual toilet, you can...Last year Raritan introduced the first MANUAL toilet designed to use pressurized fresh water--the "Fresh Head"
Fresh Head It's available as either a complete toilet or a "conversion"--everything south of the bowl, which allows you to re-use your existing bowl, seat and lid...any bowl that has a + mounting bolt pattern will fit it.
Or swap out your existing raw water electric toilet for one designed to use pressurized fresh water...Jabsco offers their 37xxxx series toilet in both sea water and fresh water versions, as both complete toilets or "conversions" (everything south of the bowl...any bowl that has a + mounting bolt pattern will fit it). Or for about the same price, you can upgrade to a Raritan SeaEra QC
SeaEra QC (complete toilet or "conversion"
SeaEra Conversion Kit
None of these should cost you much more than adding a tank and all the plumbing needed to add a separate flush water tank...which, btw, CG require that it have it's own separate fill and separate vent line...it cannot share ANY plumbing or even a vent with the potable water supply. If you've gotten away with it--or any of your other cockeyed Rube Goldberg modifications, it's only because the inspectors aren't aware of the reg...I can name several other regs 90% of 'em don't know about either.
The Raritan Marine Elegance
Marne Elegance Promo Sheet is the top-rated all china "throne" available--including the optional 4 option "Smart Flush" Panel"--for about $600 if you know where to shop.
A 3-option flush panel is also available--a rocker switch that lets you add water to the bowl, dry flush or both similtaneously is also available...this one will also work on the fresh water version (ONLY the fresh water version) of the SeaEra.
Bottom line: you have only three choices that don't endanger your health that of everyone who's ever aboard your boats:
1. Continue to use raw water to flush your sea water toilet
2. Add a separate flush water tank that has no connection whatever to the fresh water tank.
3. Swap out the toilet for one that's designed by the mfr to use onboard pressurized fresh water.
That's my $.02 worth...you did ask.