Propane Tank Switchover Valve

How reliable and accurate have you found this unit supplied on nearly all TMs? Ours on our new Elkmont appears defective and/or we don't know how to use it properly.

With the handle pointing to either tank and both tanks full and both valves on; after about an hour of use, the flag turns red. Leaving both tanks on and switching the handle to the other tank the flag turns black. Switching the handle back to the original tank the flag stays black until after about an hour of use when the flag again turns red.

After six nights of camping and driving over 900 miles with the reefer on propane, about 3 gallons of propane was used, i.e., the flag should have stayed black?