I am the original owner of this 2005 Chevy 2500HD 6.0L gas truck CrewCab Longbed with factory 4:10 gears and the locking rear diff. Stock drive train with 185K miles on it. This build is my “after 50 years old”, a lot of bolt on stuff project, but I did have to build the carpet kit/sleeping platform. Not meant to be a real expo truck, but more of a get out of the heat in Arizona weekender. Hopefully, capable enough to get far enough away from the masses….
Suspension Front: Cognito Motorsports upper control arms, steering upgrade, green keys, and fox 2.0 shocks.
Suspension Rear: Removal of the overload spring replaced with a Deaver progressive on AirLift air bags , with fox 2.0 shocks.
The misunderstanding about Arizona is that there is lots of sun for solar, but the heat protection and bad performance of the overheated panel pretty much brings the battery charging process to a halt in the hottest part of the day. My house battery is in the engine compartment along with the solar controller, so that doesn’t help. It works a lot better in the mountains and cooler temps for sure. But regardless, it eventually got the job done
Isolators do tend to eat a little bit of current while performing the job of isolating which may be the cause of the different readings at you isolator and battery.
Thats my best guess without being able to trace your wires myself or seeing a drawing of how everything is wired up.