[SatanicMechanic]

SuperTrooper is correct about the compressor, but if the propane tank on the vehicle is empty, the larger source tank will fill it depending on how full the source is. Tank equalization.
Let me tell you guys about my friend with the propane powered truck in high school. In the back of his truck he had a bike. If he ran out of propane, which he did usually at night, he would go combing the neighborhood for gas grills. When he found one, he would take the 5 gallon tank, connect it to the empty tank and take out as much as he could. He would take the empty 5 gallon tank and throw it back on yard of where he found it. Sometimes he would slip $5 with the tank but a lot of times he didn't.
He could not do this trick in winter since propane doesn't want to flow when it is freezing. One trick his dad found out was if you turned the 5 gallon tank upside down, wrapped a warm towel around it and it will flow.