The certification label on your tank should be correct.  If you are not able
to fill it to capacity, 80%, it may be that your auto-shutoff valve is
shutting off too soon.  There is a float lever which activates a valve to
prevent filling above 80% WC.  Sometimes the rod on the float arm gets bent
and could cause shut off to happen too soon.

Here is my vanagon LPG installation
http://www.telusplanet.net/public/gary2a/rack/propane/propane.htm

Gary Lee
http://www.telusplanet.net/public/gary2a/rack/vanagonrack.htm