Insert a DVM in the battery ground lead. Wait three minutes for modules to hibernate. What is the current reading?

Anything over .055 amp (55 millliamp) is too much. I suspect you will see at least .3 or .5 amp.

Remove each 50 amp fuse in the little box next to the battery - did that do it? One if brake controller, one is fan controller.
If that doesn't do it, yank the 200 amp fuse - if that did it, start pulling fuses in the underhood fuse box after pulling each fuse, IF the current goes UP, wait three minutes then look at it again. Eventually you will find the cause.

If you can't get current down after pulling fuses, or if pulling the 200 amp did NOTHING, yank the big battery cable off the back of the alternator - did that do it? Have it rebuilt, it has one or more leaky diodes.

Or take it somewhere that the 'mechanics' know less than you do and pay out the *** for them to guess. Virtually NO ONE understand basic electricity these days. It's sad, but the average American is dependent on others at every level of their existence. I, personally, was raised to learn to do stuff on my own. It has served me well.

End' of advice and rant.........