I was reading this post that might relate to your situation...
"Since the RV in question is a motorhome, not a trailer, I suspect the tank is a frame mounted ASME tank, which is not subject to the same rules as the DOT tanks used on trailer, gas grills, etc.
As stated, the best bet is to take it to a propane supplier, such as V-1 Propane, Ameri-gas, etc. and have them check it. Do NOT take it to Flying J, or U-Haul, or the neighborhood convenience store. Those people haven't a clue!"
Of course this will put you in the situation of where to fill the propane tank and keep from being told they can't fill it????
Needs more research...
Roy Ken