The attendant that filled the tank , may have left the service valve off , It's the law they have to turn it off while filling.
Open the tank access door , the service valve knob , looks like a house water tap , open it slowly , because there is a safety valve in the tank for excess flow .
With the lines drained it's easy to trigger the excess flow valve ; so when opening the valve ,if you hear a thunk , that's the excess flow valve closing and you'll have to close the valve and start over.
Back pressure in the propane lines keeps the excess flow valve open , empty lines= no back pressure.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.