I referred you to the wrong page in the manual...should have told you to check page *2* for a drawing showing the location of the solenoid. If the toilet is below the waterline, it can't go below the sole. There needs to be a vented loop in the intake line that's at least 6-8" above waterline AT ANY ANGLE OF HEEL, which on most sailboats would put it 2-3 feet above the bowl. The solenoid is installed in the loop. That would put it where Marlow thought it would be. If it's not, or somewhere else that puts it where it belongs, your toilet wasn't installed correctly.
You should find it if you trace the path of the intake line from the toilet.
However, there are TWO versions of the Jabsco Quiet Flush...you failed to mention which one you have. If yours is the pressurized fresh water version, it will use a different solenoid--not the siphon break that's needed with the remote intake pump, but one that acts like a faucet--it opens to allow fresh water to come into the bowl, closes to shut it off. It can be just about anywhere in the flush water supply line. Typically the flush water line is teed into the cold water line to the head sink. If it's in an inaccessible location now, you CAN replumb it to a location that is accessible. You'll find the manual for it here:
Jabsco Quiet Flush FW ownes manual
If none of this helps, I suggest you talk with Jabsco...Call Paul Campagna 978-282-5246