Hi,
Modern propane tanks are required to have excess flow valves installed on them. So if you exceed about 250,000 Btu's per hour will trip the valve, so will opening it to quickly.
Try this. Shut both valves, and make sure that everything is off inside the RV. Then turn on the valve slowly (passenger or drivers side does not matter). Then shut it off to make sure that the excess flow valve is re-set again with some propane pressure already in the low pressure regulator. After that, turn on the valve all the way, and it should be ready to work. Then you can turn on the other valve when you want to, and the switchover regulator should change everything automatically.
Fred.