You fill from a nurse tank, which is a tank that has what is called a wet leg, or a connection on the bottom of the tank. Basically the wet leg is a tube extending to the bottom of the tank with a valve above. Pressure within the tank forces liquid propane up the line and into a hose which hooks to the tractor tank. A fitting can be removed on the bottom of the tank to accommodate a valve and hose, but I'm of the opinion that isn't very safe. There wouldn't be a safety valve in it to stop excess flow should it be broken off.
Liquid propane begins to boil at -44º F so anything above that creates vapor (gas) similar to steam coming off of boiling water.
The larger nurse tank has more surface exposed to the elements so boils liquid more readily than the smaller tank of the tractor. When the pressure equalizes liquid from the larger tank ceases to flow so vapor pressure is bled from the smaller tank so that liquid can continue to move fill its space. There are alternatives, but that is the general way farmers fill their tractors without a pump or compressor.
To relieve the pressure within the smaller vessel a special fitting is applied to the tank vapor return fitting and when screwed partially down it allows vapor to escape. Generally a hose is attached to carry the vapor downwind.
As I previously mentioned tanks have safety valves which snap shut if there is a large amount of flow too quickly. When beginning to fill if you open the hose valve too quickly sometimes the safety will close off the flow. You can generally hear such. Just close the valve to allow it to reopen and proceed again.
Cheap, yes. A valve and hose and a couple of other fittings will allow you to do the process. With a bottle POL fitting you can also fill bottles. Oops, that is old school for the older style valve. The newer ones do have a different fitting though I think the POL one will fill those too.
Have filled my share of tractors; G 705, G VI, M-5, a Massey Ferguson, pickups, delivery trucks, and a few million gallons to homes and oil field engines. I used to work for a dealer (7 years), but dad and I had the tractors.
I hope I wasn't too in-depth.