Legal disclaimer: Don't do anything without proper training and equipment under safe conditions.  Not that a farmer would ever do that. 

Do you have 33# tanks or 44#?  Because the 33#, the common ones, hold under 8 gallons while the 44# hold 10 gallons;

I have 33# motor fuel tanks for the forklift and manlift, and a 44# when I feel like a glutton for punishment carrying it.  They fill just like your grll cylinders, just need a forklift adapter.  I have one that has a POL female to forklift with the screw on nut (can't remember which is considered male or female).

Most have the ACME fitting to fill from a propane truck.  But my 44# has a POL service valve like and old grill cylinder or a 100# cylinder.  This lets me use it for portable heat, like a torpedo heater