I used to have a propane tractor. I had what they called a wet line (cost a little more than $100) to connect from the 500 gallon tank to the tractor propane tank. Now the tractor propane tank had a vapor valve that one opened to release pressure to allow liquid propane from the 500 gallon tank to go into the tractor tank. When liquid propane started coming out of the vapor valve, the tank was full. Took about 10-15 minutes.
I don't know how they fill the 20 gallon tank, but the local propane supplier sold me the hose and showed me how to fill the tractor. Probably not inherently any more dangerous than filling a tractor with gasoline (which means one better be careful), but more time consuming. Also, I could not take a five gallon container to the field when one ran out of propane, although one could slowly take propane from a 20 gallon container and hose it into the tank. Fortunately, I never has reason to try that, but I did pull the tractor in a couple times when I misjudged the tank level due to a faulty gauge.
Also, once the tank is filled, in the process of removing the hose, one has to release the propane under pressure in the hose. That is a bit of a thrill the first time as the vapor shot out 20 feet or so. One hopes there is never a spark around, but that is also true with gasoline vapor.
My 2 cent. You have to decide if it is worth it.
Bookman