Problem with keeping both valves closed and then dumping, is, it can cause issues with the septic tank. A tank has multiple chambers, to allow solids to precipitate out and the liquids to flow to the next chamber. When you make a big dump, you chance churning the first chamber thus putting solids into suspension. This allows solids to flow into the second (or third) chamber and into the field. Solids will clog the field and potentially lowers it's capacity to drain liquids.
I am currently at my daughters home, on a septic. I keep the gray tank open and drain the black tank every 3 or 4 days, flushing with the black tank flush. The gray water is slowly (as used normally) discharged into the first chamber and doesn't overload it. The solids are dumped at a relatively low volume again so as to not overload the first chamber.
Good Luck.... PS I don't use chemicals typically, (we dump often as above) but when I do the RidEx RV is a good product... it is similar to the RidEx that is recommended to be added to the septic tank to aid in "digestion".
2011 Toyota Tundra double cab