Marine/RV holding tank chemicals. Pump out frequently, not JUST when its full. Contents should not sit in the tank for more than 5 days. Before pumping out, run a bunch of fresh water into the head and pump it back to the holding tank. This way the only thing in the sanitation lines is fresh, clean water. Once the pumpout is done, run more freshwater and the marine/rv holding tank chemicals.
We use about 2x the recommended amount of holding tank chemicals. Our preference is Dometic products. We avoid the 'All Natural' crap.
If your boat has the black sanitation line, then replace it with the white, non-permeable kind. The older black line is permeable over time.