Very simple solution if you only have 1 dump outlet. Get an inline cutoff valve(I call it main drain valve) and install it on the dump outlet. Close it and open both gray tank valves. The you have the combined capacity of both. Just remember to not open the black tank valve. To drain the black tank close both gray tank valves and then open the main drain valve dumping what is in the lines, then drain and flush the black tank and hose. Close the black tank valve, close the main drain valve, and open both gray tank valves, and you are back in business.
Also a main drain valve is nice if you have a seeping drain valve that fills the lines while traveling. It's much easier to fix if it seeps than fixing the covered up seeping valve. And keeps you from getting whatever dumped on your hands when the drain cover is removed.
Here is what I am talking about,
https://www.amazon.com/Valterra-T58-Twist-On-Waste-Valve/dp/B000BGHYJS It goes on the drain outlet and then the cap or elbow and cap.