Keep in mind there are some tank additives which if put into a septic system in high enough concentrations can inhibit the natural process of the tank. If you plan to do this then you should use no tank additives at all, or use one of the bacterial products, which are readily available and which will just boost the septic action.
There is one other thing to consider, if you do this often and that is the size of your waste tanks and of the septic tank. If your septic system is running at near capacity and you put in another 40 to 100 gallons from the newell, you could flood the system and so push liquids out into the drain lines before it has completed the septic action.
1983 Newell Coach