The holding tank only holds 9 gallons, and if you can go for more than 7 days without pumping out--OK Generally a couple will produce more than 9 gallons of sewage in a week....but we always pump our boats and RV's out at least once a week.
We have used the enzyme treatment systems with good success. Also there is the "good bacteria" treatment. Be cautious of using toxic chemicals, which suppress odors if they are going into a septic system.