before disconnecting anything turn the valve off at the tank...after that just pull it all out..there is no possible way to blow yourself up..
as for emptying the gas from the tank?..i had a special hose made up and i empty the gas from my tanks into other useable bottles..
in nsw to send lpg tanks to scrap you have to empty the tanks of gas and remove the float mechanism and get a bg holesaw and drill a hole in each end..
easiest way to get rid of unwanted lpg gear is to put it on ebay as a local pick up for 99 cents and let the buyer have the problem...or ring a couple of gas places and offer it to them ...cheap..