<<my mileage suffered. The usual 18 or 19 mpg on the highway sank to a
poor 15 mpg >>

This will happen with the engine being loaded with running the
compressor.
Not an uncommon happening.

<< the AC felt great and I had no problems except for a couple of times
when the engine died at a stop.
I'm thinking the idle needs to be adjusted since it only happens with
the AC on>>

Is your AC idle relay hooked up, and functioning?
This compensates for the drag on the engine at idle .

You can bump up the idle a bit, but then it will always be high with the
AC off.
Check the relay for proper operation.

Don't expect too much out of the system as long as it more than likely
has 134-A in it, even with two evaporator assemblies in the Van.
Your immediately losing 20 percent of the systems cooling capacity, and
efficiency with the 134-A.