I agree^^^^^

Is this 100% factory wiring?
No electic fans, fuel pumps, winches.. ?

If so, my first suspicion would be the battery. if its in low state of charge, or has a plate detach, it would suck current like crazy.
Second suspicion would be some sort of short. Maybe cross wire or maybe with chassis. The fact the instrument gages are sometimes effected really has me concerned about that.
There were some options that drew power from the battery side of the ammeter. I think some years/models a clock option was tied in there.