The holding tank is a 'fermentation vessel' when filled with 'waste' and unless the tank walls are 'transpiring' or permeating the noxious gases right through the tank walls (quite possible with thin wall fiberglass tanks), the following may be helpful:
1. Diet .... lessen the consumption of sulfur bearing compounds: Onions, Garlic, egg yolks, red meat and fish, BEANS for a few days before and when you're on the boat.
2. Increase the diameter of the vent line and have that vent line OPEN to atmosphere - OR install another/larger vent line from the top of the tank to the atmosphere.
2a. Consider air-injection into the tank: just a small 12 vdc piston pump that delivers its air to bottom of the tank (to a 'porous stone' to make the air bubbles as small as possible). This will continuously 'sweep' the gases caused by anaerobic fermentation and dilute them significantly. The injected air will change the fermentation from anerobic to aerobic and the stink will be much less. The small amount of continuous forced air simply goes out the vent. This is 'aquarium store' stuff but will need to be modified to 12vdc.; there are 'marine versions' (relatively expensive) of holding tank 'air-sparging' systems.
3. Flush with only 'fresh' water ... do it 'by hand' (shower wand, etc.). DO NOT connect your fresh water supply to the head system!!!
3a. When leaving the boat, always be sure that there is fresh water to visible level in the 'bowl' ... this will seal the head (and boat) from retrograde gas passage from the tank, through the head and into the boat - a gas trap. Also, dont totally pump the bowl dry when using as if there is a faulty valve in the head the gases can easily 'backfill' into the boat FROM the tank. If the partly filled bowl always drops its level, then its time for a head 'rebuild' - to reestablish the 'water seal' between the tank and the head.
4. Clean the tank and its hosing with commercial boiler descaling compounds such as RydLyme™ - inhibited/buffered hydrochloric acid that will remove the 'porous' carbonate build up .... caused by the reaction of urine and seawater. RydLyme is also used for cleaning engines and boat heat exchangers. Apex - RYDLYME Marine is a safe, biodegradable, non-toxic, non-corrosive marine descaler. RYDLYME Marine is designed to dissolve the toughest water scale, lime deposits and rust deposits.
The porosity of the formed carbonate is a 'whopping
' stink reservoir ..... if you ONLY flush with fresh water, there will be minimal carbonate build up
5. Alternatively to all the above, Change out the ENTIRE 'head system' to a 'Composting' Head ... but you're going to have to lug/carry/haul out all the 'contents' instead of pumping it.