My thoughts are that you have a bad belt and or tensioner that's letting the belt slip. It could also be a seized fan clutch overloading the belt.
A slipping belt causes the alternator to slow down and not charge.
If you want to disconnect the chassis to house charging feature, find the isolation solenoid by having someone push the aux start or boost switch on and off.
While that's being done, listen for it clunking. Follow the noise to it, and once found, disconnect one of the small wires from it and tape it up.
I should mention that you are the only post I'v read, on this site, complaining of this problem.
Should look like this.