I have had good luck on my SM 2000 minimizing any smell from the
toilets by closing the sea water inlet and running fresh water
through the strainer from a dockside hose when I am ready to leave
the boat for any appreciable length of time. I turn on the engine,
the generator, the A/C and have someone flush each toilet for 30
seconds or so while the fresh water is flowing throught the sea water
inlet. Thus, all the systems have been flushed clean of salt water.
I have left the boat for as long as 2 months using this method and
there was never any smell on my return. I also recommend you pour a
bottle of white vinegar down each head about 1x a month. According
to Cruising World magazine, it should take care of calcium deposits
in the toilet lines (which once caused me a long afternoon of
disassembling the line coming off the head and breaking up the
calcium deposits plugging it up). Don't leave the vinegar standing
too long as it can reportedly do damage to the rubber parts in the
pump if left in contact over time. I hope this is some help.