Whatever you do, do NOT connect your toilet intake
line to your fresh water plumbing! NO toilet--manual or electric--that's designed to pull in raw (lake, river, sea) water should ever be connected to the fresh water plumbing...it cannot be done without risk of polluting the fresh water supply, damage to the toilet, or both--and every mfr specifically warns against doing so in their installation instructions. Only toilets designed by the mfr to use PRESSURIZED flush water can safely be connected to the onboard fresh water system.Use the shower head, or pour water into the bowl using a cup or a jug, but don't have a "brainstorm" that the solution to your problem would be to connect the intake to a fresh water line...'cuz that's a MAJOR no-no. Btw, whether fresh or sea water is used to flush has no impact whatever on holding tank odor, only on odor from sea water trapped in the head intake--and MAYBE from the head discharge line too, if the joker valve in the toilet is so worn that the slit has become a hole. There's so much bacteria in waste alone that whether any more is contributed by sea water makes no difference whatever in the tank. The tank is rarely if ever the source of odor INSIDE the boat anyway...'cuz unless it's leaking, odor in the tank only has one place to go: out the tank vent. However, rinsing out the head discharge hose before the boat sits helps to prevent odor inside the boat because it helps to prevent the hoses from becoming permeated with odor.There's a very simple solution to sea water intake odor: tee the head intake line into the head sink drain line. Before the boat sits, after you've closed the seacocks, fill the sink with clean fresh water...flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink, rinsing the sea water out of the whole system--intake line, pump, channel in the rim of the bowl and the head discharge line.