Have you thought of perhaps using a screwdriver on the bleeder valve?  Seems to be the way all the propane guys do it on removable bottles (which I have in my vanagon)  You could probably replace or re-configure  the bleeder valve knob so that a screwdiver would work or maybe a socket.  Or how bout some needle nose vise grips? 

 One tool that does everything for me...the needle nose vise grip..Don't leave home without some...

I won't have another van with an 'under the chassis' tank.  Very inconvenient, and quite dangerous, in my opinion.  I would rather sacrifice some interior space to a real portable tank so that I can fill it at my convenience without having to work my vanagon up next to some propane vendor's hose.  I just make sure I have plenty of ventilation wherever I mount the tank inside.  I've seen many tanks bashed and almost breached by road debris or curbs or rocks..  I am not saying this as a recommendation.  The DOT probably frowns on it..

Don Hanson