Purging propane tank

Let's be clear on what "purging" means in regards to safely shipping an RV. Purging a propane tank or cylinder for normal use means replacing any air in the tank with propane fumes before filling. But for shipping a used tank, it means just the opposite, replacing any propane with air or an inert gas such as nitrogen. A vacuum purge can also be used and is preferred since it allows the tank to be refilled with propane without a pre-fill purge. The difficulty comes in finding a facility that will do a "reverse purge". I suggest checking with an industrial gases supplier to see if they have the needed capability. That would be supplier of welding and/or medical gases, etc.