Obviously your toilet is wired to the start battery...which isn't that bad a plan, actually, because electric toilets need to be on their own separate dedicated circuit, shared by nothing else that can reduce power to it...low voltage will destroy any electric motor and is the most common cause of electric toilet failures..Since it's unlikely that anyone will be flushing the toilet while the engine(s) are being started, I wouldn't change a thing...UNLESS the possibility exists that the toilet could get enough use during an extended period on a hook to deplete the battery. In that case, best to run a new circuit to a house battery.
While I have the floor... IMO, the toilet that landlubber guests will use should be a no-brainer to flush 'cuz guests cause at least 90% of toilet problems. "Push the button, count to 3" is something even great aunt Martha can grasp...the most common landlubber reaction to "move the lever to the "wet" side, pump 6 times, move it back to wet and ..." is glazed eyes and a wet wipes clog you have to surgically remove after they leave.
As for noise, today's electric macerating toilets are a LOT quieter than they used to be, especially those designed to use pressurized fresh water. The Raritan Marine Elegance is the quietest marine toilet on the planet. Their SeaEra QC is a close second.