That is a good question and one I cannot answer. I wonder if anyone else can chime in on this. I do know that one can purchase horizontal mount tanks on the vintage trailer supply website...I assume this is made for the horizontal tank. It does have special brackets welded on the hitch to support the tank.
Some cylinders ARE designed to lie horizontally for RV's and forklifts. If you look at these cylinders closely, you will see that the relief valve is mounted in the upper portion of the vapour space so that it will not vent liquid. The service valves on these tanks has a tube inside that curves upward to the top of the vapour space for the same reason.
Too make a long story short...Any appliance like a BBQ or stove that might be commected to a normal 20LB cylinder should have the cylinder always in an upright position!
cylinder contains liquid and vapour. We only fill cylinders with liquid to 80% to allow room for the propane
to grow if the temperature increases. This is done because we don't want liquid to be in contact with the relief valve, which is built into the service valve. If the relief valve were to vent due to an overpressure situation, it will vent vapour. Not necessarily something we want to happen, but if it does, we have a bit of propane
vapour in the air. One of the properties of propane that you must be aware of, is that propane expands 277 times when it changes state from a liquid to a vapour! So if the cylinder is on it's side and vents liquid, that little bit of liquid just made a VERY LARGE amount of propane vapour!