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Just running the tank dry will leave it full of propane vapor, still an explosive situation. As Jeff said, the tank would need to be purged of all vapor and filled with a non-flammable gas like nitrogen or even air. Before filling the tank again, it will need to be purged of all gases before putting in propane. Any propane dealer should be able to do both of these tasks.
? Propane tanks - Vehicles with propane tanks for auxiliary uses must be delivered for shipment with the propane tanks empty of any liquid or fumes. The empty tank must be tagged and certified as "gas free" by an authorized propane dealer or service company. We recommend that you verify the cost of emptying and certifying propane tanks. It may be less costly to purchase a new tank at your destination. [My emphasis]
The agents give you that in writing? They would be authoritative. However, to empty the tank you should go to a propane dealer that can handle it, and you should ask for a certificate....just in case. I am an old bureaucrat. If the paper can be got without a hassle get the paper. It can be amazing how short memories are when push comes to shove.
Faithfully following this thread, we attempted to contact 10+ propane places in the Baltimore area before shipping our RV. None would do it. Finally one called me back and saying they could help out. We're here now patiently waiting for our tank to burn itself completely off before they can inert it with nitrogen. The tech explains that all of the liquid needs to be gone before they can overcome the native propane pressure in the tank to inert it. Hopefully will still get the rig to the doc today...