Jim,
I have a '91 190e 2.6 and went through the same thing a few months ago.

After replacing some of the vacuum actuators and the potentiometer like you did everything was holding a good vacuum. The switchover valve was also operating correctly.

However, things still weren't working and I discovered that one of those little rubber elbows that connects vacuum lines was cracked. Check all of yours.

Vacuum supply starts at the intake manifold then runs to a check valve then to a container in the right fender. The vacuum then runs to the vacuum switchover unit which is right behind the rightside vent behind the dash.

I know the system changed alot over the years but that's how mine was set up. Hope this helps.

Bruce