Quick background on LP (liquid propane).
LP (sometime called LP gas - LPG) is stored in your tank is always part liquid and part vapor (gas). The gas, and therefore the liquid, is under pressure. Some large tanks dispense liquid, but the typical tank and RVer would run into dispenses vapor.
If you hook two tanks together with a hose you will wind up with two partially filled tank as the pressure will equalize. This is why it is difficult to refill disposable bottle.
The pressure in your tank varies with temperature. If you want to transfer the LP, you need different pressures. Cool the receiving tank in an ice water bath. If not practical, heat the full tank with hot water. Placing the full tank in the sun and the receiving tank in the shade DOES WORK but it takes a long time !
A temperature difference of about 20F should give you about 20 additional psi.
