You started with Freeze 12. By most of the regulations I've seen for
these propane derivative refrigerants, to be legal, they system should
have first been converted to an R134 system (and that means the
lubricants and connectors) first before Freeze 12 can be used in the
system.

Then you switched to R12. However, unless the system was cleaned out
and new R12 lubricants put into the system, you could have been mixing
incompatible refridgerants/lubricants.

Switch everything over to R134 (including the connectors for hi and low
sides. Make  sure the proper lubricants are in the system that are
compatible with R134. And sufficient amount of those lubricants (the
length of refridgerant lines in a Vanagon means that you'll be using
more lubricant in the AC system than is typical of other vehicles.

Once all the above is done, you should be able to use R134 or, after an
evacuation of the system, Freeze 12, or Duracool without problems.