There ARE differences, HD5 is the propane industry standard that is required for forklifts since 2004. LP gas does not "go bad" like liquid gasoline that is not stored in air and moisture tight containers.
it is possible to get contamination into the LPG tank that would in turn clog the LPG systems filters (as long as the filters were still there) or the LPG regulator. it is also possible that the LPG regulator is a problem from contamination or non use causing seal failure
Your problem does not sound to me like "bad gas", as much as a failure in the fuel regulation and delivery system.