This system has worked well for me and I have never ran out of propane doing so.
Starting with two full tanks I shut off the back tank and run off the outside tank. When it is empty I open the back tank close the front and turn the flipper valve to the back tank. Within a couple days I get the outside tank filled and repeat. After doing this for several tanks I will place the newly filled tank at the back and use the now partially full tank until empty and repeat.
Yes it's possible to run out of propane at night but I usually had small space heaters going. I have had it happen and it's not that big of a deal on the rare occasion it happens. Better than running out completely!