DP Charging Problem

Wolf's on top of things as usual. I'd also check alternator belt, if it's independent of other belts. It almost sounds as if it's slipping at higher RPMs. Just a suggestion.
Scott

I would start by cleaning and tightening connections at alternator and battery isolator.

THEN, You really need to troubleshoot the charging "system".

Pay particular attention to the "sense" wire (which you will have if you have a diode-based battery isolator). It is a small gauge wire and a little corrosion, loose connection, etc really does strange things to voltage.

I would not drive until this is fixed or a work-around is wired (if you have to move, let us know and we can talk you through some pretty easy work-arounds). You can damage engine and transmission computers with low voltage and/or they can leave you on the side of the road (timing theirs, not yours)!

Check the engine batteries with a voltmeter before cranking and then after...you should get a rise in voltage to above 14 if the alternator is charging.