Zoltan was absolutely correct.  The propane fittings are just normal right hand threaded fittings so turn anti-clockwise to loosen or remove.

I did try that at first but I got nervous when the whole assembly started to move!  The trick was to grip the body of the assembly (to the left of the regulator) with a pair of channellock or adjustable pliers to hold securely and then remove the union nut with an adjustable wrench.  I did use lots of Liquid Wrench first and I cleaned the copper piping so the union nut could more easily move along the copper piping.  The nut released fairly easily once I had the whole assembly stabilised.

The rest of the tank was quite easy to remove.  Just remove the 2 outboard nuts completely and loosen the 2 inboard nuts.  You don't have to completely remove the inboard nuts but you do have to loosen them a fair bit.  Then gently lower the outboard part of the propane tank to clear the body work of the vehicle and then slide the tank off the inboard mountings.  

The tank was not as heavy as I thought it would be.

There is a gotcha though if you have air con.  The inboard nuts are hard to reach because of the flexible air con pipes.  I had to first remove 2 clamps that hold the air con pipes to the underframe of the van.  I thought it was a small metal nut holding the clamp that the air con tubing passes through but it's part of a plastic fitting which is shaped like a nut.  I broke the first before I realised what is was.  I think the trick is to loosen the plastic nut gently about 1 cm and the plastic clamp is a 2 part fitting that clamps together.  Pinch the inner part together to release the lock and gently remove.  I also used lots of WD 40 first.  The plastic clamp actually fits over a threaded stud about 4 cms long attached to the frame.  That's what I broke!  It just sheared off!

Thanx again to the list and good luck to you all

cheers
Rich
90 Westy
Vancouver BC