It is a puzzle why the 75 did nothing after being reinstalled. It did have DC output in its bench test before that. No doubt the OP is chasing that down with a multimeter.
Once this all gets sorted out, I would like to see (photo please) how the PM-75 is installed so you can get at it to operate the voltage adjustment pot and see the voltage changes as you do that.
Just getting at the hole to put in the small size screw driver is one challenge. Running back and forth to see the Trimetric display is another. A voltmeter co-located with the adjustable converter solves that. If you can easily see the voltmeter while twiddling the pot!
It is easy to arrange for all that in some RVs and not so easy in others, depending on physical layouts. The OP could demonstrate how he does it, which would help others. With adjustable voltage converters becoming the new thing, we are all going to have to solve that somehow.