Trouble shooting the cause of Charge Line blown fuse

Is this a new problem? That is, was it working at one time?

If it was working then you need to take the connectors apart for an inspection. They often have loose wires that short and chasing that with a voltmeter may be incredibly difficult.

If it only does it with the RV plugged into the truck then inspect the trailer plug closely. I mean take it apart carefully. Heck, a simple screw rattling around inside can drive you crazy.