I'll give you two possibilities.
First, when having electrical problems on vehicles, people often fail to check their grounds. On a vehicle, the grounds are more often the problem that the hots, because they're a little more loosey-goosey in the original design that the hots. And most electrical problems in vehicles are connection issues, at least initially.
In particular, GM trucks have four separate parts that are insulated from each other by rubber bushings and the like and need to be wired together: the battery, the block, the frame, and the cab. Connecting four things to agree on where zero volts is takes three wires. One from the battery to the block. That's the big one. One from the battery to the frame. That's the smaller one from the battery to the radiator upper cross-member. The third one is from a valve cover screw on the rear passenger-side valve cover to the firewall. When that one is not there, you get all kinds of screwy stuff going on.
Second, you can replace the alternator with one that has the same housing and uses the same brackets but has higher output. That unit will have higher output all across its rpm range. We add stuff to our trucks and forget that the alternator it was originally equipped with wasn't specced with that additional load in mind. The original on the 85 C-10 was probably a 63-amp 12SI. You can get a 100 or 140 amp 12SI form alternator and just bolt it in. No bracket or wiring changes.