Most septic systems can handle occasional holding tank disposals but it seems like it will fare better with small amounts at a time. I would spread the load over a few days if the tanks are full. This gives the tank a time to settle and the leach field time to drain.
It depends on the laws in your local jurisdiction. Some places it is legal and others it is not. As to whether or not your septic tank has the capacity to hold your rig's tank contents..... Normally you will have plenty of room to dump your tanks. If not then you probably need to check your septic tank. Have fun.
Jimmy Dean Ross
1988 Newell Coach
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.