There are many chemicals available, for use in RV holding tanks, most use the word digester as a selling point. The true digester in a holding tank is water. Keep in mind if your holding tank is 40 gallon capacity, and ¼ of it is solids (10 gallon) that is equivalent to 2 ea 5 gallon buckets full of solids. It will take a lot of water to get all those solids out of the tank.
Tank Deodorant: Medium cost, masks odors, does nothing to clean your tank, or eliminate cause of odor. Available in powder, drop in packets, premixed chemicals 8oz. bottles up to larger sizes. May cause more sludge in bottom of holding tank, and may contribute to the growth of struvites in the Black water holding tanks.
Enzymes: Higher cost, helps to control odors, helps to clean your tanks, helps to eliminate the cause of the odor. Available in powder and premixed chemicals 16oz. bottles and larger. Though enzymes will help to maintain cleaner holding tanks I don’t think that in normal RVing applications, the enzymes are in the tanks long enough to do you much good, but the septic tank downstream will benefit greatly from your investment.
Probiotics: Higher cost, much like enzymes, probiotics control odors and helps to maintain a clean and healthy tank in an enclosed environment, however in most Rving applications, the probiotics are most times donated to the installation downstream from your dumpsite. In most cases the cost are not justified by the benefit.
If you are happy with the chemical you are currently using keep using it, if you would like to try a good and less expensive alternative to the above options, consider trying the following recipe in both the Black and Grey tanks each time you dump you tanks.
In a 1 gallon container (wide mouth preferred) Pour a 48 oz. bottle of Pine-Sol (any scent works). Refill the 48 oz. Pine-Sol bottle with water, add the water to the gallon jug. Then add 1 cup Calgon Bath beads.
Be sure to dilute the Pine-Sol before adding Bath beads!!!
Shake well. Add 8 oz. to black water holding tank, 4 oz.to grey water tanks, add 2-3 gallons of water to each of the tanks. Your tanks are now ready to use.
When hitting the road after dumping your tanks, add the cleaner, deodorant as above and fiIl tanks half full, allow tanks to slosh while on road, this will help keep your tanks cleaner longer.