Do you safely decant an on-board vehicle LPG tank to another vessel, be that another vehicle, an LPG pump, or an empty gas bottle?
lt’s tricky. Best done out of the vehicle in a well ventilated area as there is always going to be lots of vapour escaping. Not around houses or people.
I would release vapour pressure by undoing partially the gauge. Once this starts leaking it is very hard to stop. Don’t undo gauge too far or you can cop it in the face. Tank will freeze up and be very hard to handle until.
Once pressure is released you can screw the old tap off and replace with new one or replace o-ring (these are not just rubber). Remember that the some taps are left hand thread unless it’s a sprint valve or BRC.
Replace tap. Refit gauge (you need to replace the seal behind the gauge as it will be damaged and might not seal).
Or just take it to a gas fitter or tester and they will do it for you. There are rules and regulations around decanting. Let those who have the relevant licences perform jobs such as tank repairs. Too dangerous if you don't know what you are doing
It is a bit nervous at the thought of opening up the valve to let it vent off.