Hench wrote:
In a bout of act first think later, I purchase a backup tank of BBQ propane.

But nowhere to store this thing.

Any safety issues if I keep it under a pressured treated deck? Under the
BBQ itself is not an option.

There's not much difference between storing the tank on the deck - you
know, the one that's attached to the grill - and storing the spare under
the deck.

Do it and don't worry about it.