There are two valves inside the Roadtrek that determines how water flows between these two tanks. In my Roadtrek they are located in the cabinet on the floor on the driver's side where the bed platform starts. Inside this cabinet you also find the water pump and the hot water tank with its bypass valves. Inside are also two valves that can shut off water flow to the outside shower. Here is the cabinet once opened -
The cabinet door pulls down to open:
And inside are the valves!
The red handle valve and the small black valve are the same as in the 190 and will be set exactly the same as described in this article above!