Tank not the source of odor inside the boat
Odor out the holding tank vent line and odor INSIDE the boat are two separate issues. While it's entirely possible to have both, it's equally possible to have one without the other, and each must be dealt with separately. Odor out the tank vent line originates in the tank, not in toilet or the plumbing. However, the tank is rarely the source of odor inside the boat because, unless the tank is leaking, odor from inside the tank has only one place to go: out the tank vent. So trying to eliminate odor inside the boat by trying to eliminate odor in the tank is just chasing your tail.When was the last time you cleaned your bilge? I don't mean just threw in more bilge cleaner and/or bleach...but really CLEANED it...and rinsed ALL the dirty water out? A wet dirty bilge is a dark stagnant pond, a real "primordial soup" of molds, fungi and bacteria-some that thrive in dark stagnant water, others that just like damp dark places. The warmer the weather and water, the faster they grow. Add some dead and decaying sea water micro-organisms, dirt, food particles, rain water, wash water, hot weather and humidity, plus a little oil or diesel...no wonder your boat stinks!Sumps are another overlooked source of boat odor. Somehow it never occurs to most people that shower sumps and drains, like bathtubs, need regular cleaning to remove the hair, soap scum and body oils that collect in it the same way they leave a ring in the bathtub. And the shower sump may not be the only sump. Central heat/air conditioning condensate and sailboat ice boxes often drain into sumps instead of into the bilge. Left alone long enough, a wet dirty sump can smell like a sewer even faster than a wet dirty bilge. It's another wet dark place, ideal for growing odor-producing "critters." So if you want a boat that doesn't stink, clean--really CLEAN!--your bilges and your sump(s) once or twice a year. However, while holding tanks are rarely the source of odor INSIDE the boat, they should be at least nominally rinsed after every pumpout..and thoroughly flushed out 2-3x a year to prevent sludge buildup--and it'll go a long way to prevent odor out the tank vent, too. All that's needed is clean water. After pumpout, put 2-3 gals into the tank via the deck pumpout fitting--'cuz that sends it into the tank at the bottom to stir up any sludge...pump that out...repeat...repeat...till you're pumping out only clean water. For more detailed information about odors and how to prevent 'em, I suggest you check out the link below.