"What do you do if you are in an area where you must swap tanks rather than refill them, but the swapper is not the empty one?"
I run multiple tanks. I have 3 swappers for my bus. I have one for the truck camper. I doubt the truck camper will ever have two empty tanks at the same time.
The "swappers" are the back-up tanks to my own tanks. So I use them when my own tanks are empty. At this point in time, I have 8 of my own tanks and at least 3 swappers not counting what is on the truck camper (not sure since part of my tanks are in storage). I built my "RV" so it's not like most RVs. I use LP for heating/cooking/dryer which runs on one LP manifold system and I have an LP generator which runs on a second LP manifold system. All the tanks have gauges added to them so I can see which are empty and which are full. They also are on manifolds with shut-off valves so I can open up to 4 tanks on a single manifold and still have one (usually a swapper) as a backup for when the 4 tanks finally empty.
All my LP appliances are residential so they are all energy efficient, which RV appliances are not required to be because they are not made to be used all the time like residential appliances are, so no energy saving regulations are followed.
In my current location, legally I can only transport 3 tanks inside my jeep. When I have 3 empty, I take them to be refilled.