LP Gas problems

[Whitejack]

I hope I can explain this properly and that someone can help.
Our 31SQB has two LP tanks mounted on the front tongue. There is a "Y" connector which attaches both tanks to a single line that runs back to and services the camper. at the 2 into 1 connection, there is a valve that can be turned left or right.
Everything worked perfectly untill we ran the tanks dry.
Took the tanks to be filled and one filled properly. The other one seemed to have a faulty valve as it would not fill.
I took them back, reconnected both tanks, opened both valves and then had to turn the center control knob either left or right, can't remember and it doesn't matter.... we had LP gas again. I lit the inside range burner to bleed the air out of the lines and everything was back to working.
Went camping two weeks later and everything was working fine and then we lost gas to the inside range.
I tried everything I could think of but I cannot get gas into the camper. The one full tank is still quite full. The other is still empty and I know I need to take it to be checked out and filled but it should still work with one full tank, shouldn't it??????

[Festus2]

As a temporary solution, try moving the full tank to where the empty tank was. Yes, it should work with one tank full. Either your regulator is acting up or you are not having the valve(s) and /or control knob in the correct position for it to automatically switch to the full tank when one becomes empty.