UPDATE: 09/02/2018:
I am the original poster. I ordered the new vacuum reservoir and harness as indicated in the Ford Technical service bulletin (as suggested in this thread). But I also went forward with installing the 12volt vacuum pump (also as suggested in this thread). I connected the 12 volt pump to the fusebox (protected it with a 3 amp fuse) and connected it up to the existing vacuum reservoir, plugged the vacuum hose that originally was connected from engine.
I put a dash switch on the vacuum pump so that I can turn it off in the event I want to.
Tested in a 150 mile trip so far:
A/C vents blow correctly and much better than ever. So far, it has not blown thru the defrost vents unless it is in defrost mode.
The only drawback that I have seen so far is that the pump is NOISY. I can hear it come on and off at times. I can deal with that as long as the a/c blows correctly. We use the dash a/c and at times has saved us because the Onan was being problematic and could not run the main a/c system. Besides sometimes in the hot weather, it feels good to have the dash a/c blowing nicely up front for relief.
Will report more on this as I continue to test, but so far what Mr.Doug suggested works perfectly (although a little bit noisy).
