Depending on the age of your sewage hoses (and other equipment), the smell could well be that the hoses have allowed sewage (odour) to permeate. Test this by running a cloth along the hose and then smelling the cloth. If the cloth smells of sewage (at all), then you should replace the hose. Other things you can check or do to resolve the smell issue are:
1) Ensure that the holding tank
air vent is large enough (at least 3/4 inch) and that it is not blocked. Sometimes insects nest just inside the vent hose near the hull
exit point, or maybe the tank was overfilled with sewage, and this has dried and blocked the vent. Your tank needs air so that the "good" bacteria can flourish and the bad (anerobic) bacteria don't.
2) When you are not going to be using the boat for a few days, flush the head well with fresh water (usually just before pumpout). After pumpout, flush the tank well with fresh water through the deck
fitting and pumpout again. I do this twice each time.
3) You can use a tank "chemical" that is enzyme based to help the bacteria do their job of digesting the waste.
4) Always pump the head in "dry mode" for at least 40 pumps (more if you have a long run from the head to the tank) after using it. Failure to do this allows some sewage to remain in the hose causing premature permeation and may even allow some backflow into the head if the joker valve leaks
5) The vinegar idea described by other posters is a good one. Vinegar helps to dissolve any crystalization that can build up in the plumbing
from the interaction between urine and sea water. This soaking and flushing
with vinegar followed by a fresh water flush will help with that.
6) I advise against using vegetable oil
in the head, as this can encourage the growth of the anerobic bacteria (smelly bacteria) in the holding tank
. Instead, take the pump apart (at least twice a year) and use a water resistant grease (winch grease for example) to lube the pump. The Jabsco
pump takes about 5 minutes to lube in this manner, it is no biggy.
That is all I can think of for now, good luck,