Electrical problem

Sounds to me like something is shorting out. During the night run does it happen when hitting some good bumps in the road? You can draw a lot of amps without blowing a fuse especially if it's only for a short period of time.

 

I went back and re read your post.
The voltage dropping down like that does sound like a short (not a dead short but something drawing a lot of current)
Still could be the voltage regulator. What bothers me is that the amp meter sits to the negative side. Being negative it's basically telling you how much current is being drawn with the alternator not running.

I had a weird problem like this a while back on a 387 Pete of ours and it showed low voltage (don't think it has an amp meter). When I did my voltage test with a digital meter and the voltage at the alternator was about 14 volts. When I tested the dc bus it was like 11 or 12 volts. This was weird to say the least because the batteries never discharged and nothing bad was happening because of it. Anyway, long story short I found the specific wire that was feeding the bus from the batteries and simply loosened and re-tightened the connection and it fixed everything. There was a slight amount of Resistance in the connection at the battery and that's all it took to skew the readings. Usually, when you have a bad connection and you try to draw any current through it it will open but this bad connection was allowing enough current to be drawn so that everything worked fine. It showed up as Volt meter dropping a couple volts inside the truck.
Good luck

 

Keep us posted?

 

Did this start before or after new batteries? Had a similar problem one time,would have sworn was bad regulator. Turned out it was a bad battery,when it finally shorted out,it swelled to the point had to pry end battery out to remove.

 

Check the back side of fuse panel for corrosion. Some would have crazy problems,with lights and blower motor.

 

you have short connection