40 Pound Aluminum LP cans
If you're referring to the horizonal LP cans, these have been exempted from OPD retrofits. Contact Airstream, they will send stickers for the cans. If they are vertical cans, Airstream has the correct/compatable OPD valves and is selling them at a reduced cost.
This past weekend, I took the A/S to a large, rural LP gas center. The gas was $1.50, the empty tank held 38 gallons. I was told to tell the cashier, my horizonal can was a forklift bottle. Their standard charge for forklift bottles was $14.00. I asked if they were sure about this and they said Yes! WOW, I'll go back!