Regarding picking up the propane at the self-serve ( Home Depot/7-11 )I would go broke as they are expensive. When you have the Tank filled you normally receive the full amount ( 20, 30 or 40 pounds) where at the self serve provides 80% or 16 pounds in the 20 pound tank at a higher cost.
On the tank fail-over operation, Winter operation both tank valves are open. Summer time normally I have one tank off with tag stating Full & date OR Empty & date. Never been caught without Gas.