Hi Ellen,
This issue was answered well on May 5th (Techies Page). The "Search" box is quite useful. I copied the post & have it in my van (J.Discovery). Here it is -

"the pointer at 12 o'clock = both tanks off , pointer at 3 o'clock = front tank , pointer at 6 o'cclock = both tanks on, (water being drawn from both tanks at once ), pointer at 9 o'clock = rear tank.
When trying to fill one tank only do not have both tanks turned on as you will find that one tank will slowly decanter into the other, also do not travel with both tanks turned on as if one should be damaged and lose all its water the other tank will also lose all its water, not good - so far I've lost three tanks to stone damage over the years. and they all had good stone protection fitted."

All the best in your travels.

Warren