Which valve is it?  The fill valve?  No problem.  It can be replaced with
any normal run of the mill fill valve.  The difference is that the normal
fill valve doesn't have the overflow built into it as the VW one did. When
he replaces the valve have him put a "spitter valve" in the upper part of
the tank.  There is a place where this can be added in.  If you have a
problem getting this done let me know and I'll dig up the parts my propane
guy used.  Whole bill, including new fill valve, new "spitter" valve and a
pressure test and cirtification stamp cost me $42.00 the gas was
additional.
Wendy 
'85 Westy,   "S-CAPE"