That is the correct way to bleed out an LPG line. If the truck does not run you can crack open a LPG fitting.......NO SMOKING.......
Don v;
The equipment manufacture's and also the NFPA instructions are "Close tank ***, start and run engine, and then consume LPG in the fuel line, engine will stop, diconnect fuel line from tank using Personal Protective Equipment." After finishing work reconnect fuel line to tank using PPE, open tank *** - slowly - and start engine, check for LPG leakage using soapy water after assembly.
Safest is to disconnect the tank and run the truck till it shuts down.
I usually push in the valve at the propane connector with the rubber coated handle of a pair of channel locks. Just make sure it's pointed away from you. Or you can crack open one of the lines at the lock off.