The standard bbq tank are 20# tanks and commonplace on many RVs. When they are exchanged they are not fully filled and will contain 15# or thereabouts. We always have them filled where propane is available, but over the last dozen years we have run dangerously low on a Sunday or where there was no propane refill service in the vicinity which resulted in using the exchange method.
On both occasions exchanges were from Walmart and their low everyday pricing was within a couple of dollars of what 15# of propane would have cost at our local propane company. I don't fret the small stuff, exchanged tanks were in better condition, we were able to stock up on supplies, and on both accounts, we were able to continue our extended off the grid trips without issue.