I had a horizontal tank on an old '94 pop-up. Same scenario where a hardware store would not refill due to age.
A local propane shop did a re-cert at no cost and did so because it was in good shape. Because it was always inside the camper storage compartment it did not show the age, rust or other wear indicators of say a grill tank that is exposed to the elements and dripping grease.
Sold the TC, previous owner of 2 NorthStar pop-ups & 2 Northstar Arrows...still have the truck:
2005 Dodge 3500 SRW, Qcab long bed, NV-6500, diesel, 4WD, Helwig, 9000XL,
Nitto 285/70/17 Terra Grapplers, Honda eu3000Is, custom overload spring perch spacers.