nice info Joe. I copied it for my knowlage.
is it 'cuz condensation will consentrate to the bottom to much on a large tank if it were vertical, or maybe 'cuz it would be to tall in back of a truck??
In really cold temperatures the liquid propane has a harder time turning into a gaseous state so the horizontal tank gives a greater surface area at the top of the LP for this to happen. That way the furnace (or other propane applianices) won’t use up the propane gas faster than it can convert from LP.
In really really cold areas they install warming jackets around the tanks also for this reason.
good info Paul, thanx.