I would call them and tell them that if they mess with my tank again as in shut off the gas, I will be up with my baseball bat and work on their head a while and then sue them for any cost associated with frozen water lines. (you really should shut off your water and drain the lines if you are not using the cabin in winter to avoid just the possibility of freezing)
If you have access to natural gas, pay the cost of replacing the jets in all the appliances, you will save that in one month of use just in lower fuel cost. The propane company should not be messing with your tank and I would threaten them with a lawsuit if water lines froze because they cut off your propane. At most they can do is refuse to sell you any more gas, but coming on your property and shutting off your gas on a tank that you own is a good way to get shot if it were my property and I caught them trespassing AND lets call it what it is "willful destruction of property" which could happen if your heat was turned off and the water lines froze up.
I can understand them requiring testing of old tanks but only if the current test was expired and all the old fill valves are required buy law to be changed out to the ones with the safety shut off check valve. I don't know if this could be changed safely without the tank being empty unless one bled off all the pressure which would waste a lot of gas and the liquid would continue to boil off till the new valve is installed and blocked in.