Electrical issue

Hey all, I'm new here. Hope I am not posting where I shouldn't but was looking for some advice. I am owner operator and also work on trucks for fellow drivers on occasion. I have a 94 Ford L9000 with Cummins L10 (Mechanical). The last couple months I have had a problem with my truck. Most of the time when I start it, the volts on the volt meter shows between 13 to 14 volts. I will be driving for maybe 1/2 hour or so and then my radio will start changing from the radio over to something else like changing to the Ipod mode without me doing anything. And my cb will go to where I cannot make out what anyone is saying and they cant make out me. All garbled. Like the volts are dropping. Its a mobile home toter so I have a rotating amber light with a few small led lights. I have changed out 2 alternators on it and same thing. If it could be a wiring issue, where do think It could be? Not the greatest with wiring but need to get this resolved. Thanks for any help.

 

Multi meter reads 13.7 Volts at Alt. But that was just after I put on the latest new Alt. But that's what I think I need to do is to put it on when the problem is going on. Only problem is, most times when this has happened I have an oversize trailer hooked up and not a lot of places for me to pull over sometimes. But I think when this happens, I will have to make it a point to pull over and put the voltmeter on and see whats going on. But I did it once before and it did show like mid 12 volt with the last Alt. But what else could be causing this. Its the exact same thing with the new Alt on it. Brand new belt and I hear no slippage while driving.
That was my first guess was a bad ground somewhere. I really need to get a wire schematic of the truck. Im gonna try calling a guy I know well at the Western Star dealer where I get my parts to see if they can get me one. I put a new frame to cab ground wire on but I guess that did not do the trick. All main cables and battery terminals are clean and nuts are tight. I'm guessing this has to be either a bad ground or a bad wire somewhere. But what is my best approach with trying to track the culprit down. I know my way around the engine fine but when it comes to electrical, I might be missing most of my hair by days end.

 

Also, wondering if while this is happening, if I were to pull the fuse to the cb and radio, most likely the same and see if that cures the problem. Then I would know to look on that circuit

 

So now have a new problem. I need some help diagnosing. I came out at 5am to go to work and turned the key on and Zero power to the truck. Nothing at all. Obviously I was limited what I could do in the dark but I took my multimeter and the batteries showed 12.5. But the key and anything else in the cab I tried like lights and such, nothing would work. I put a battery charger on it and still nothing. So gave up until it gets light, which about now. I am going to check the battery connections when I go out but they looked ok. Just so I know, could someone explain to me how the power goes in the truck. Does main positive cable from battery go from battery to starter, then starter to somewhere that goes to cab? Not an electrical guy which is not good when it comes to this

 

Found the problem. Took me most of the day. This year truck, 1994, has two 14 pole Stud Junction Blocks mounted on the firewall up top. I moved some wire loom to find two wires that the terminal ends corroded so bad that they broke off. The one was a 12ga hot wire. It had another wire behind it that was on the same stud that also broke off and the terminal right next to it was tied into it with a metal plate that also had 3 wires that all went into the cab threw the firewall. As soon as I touched that hot wire to the other wires, the buzzer for the key went on and everything in the cab that should power up, did. So looks like that was the issue. So now just have to go find a new junction block tomorrow. It was a 14 stud block but there were only 8 wires on it so hopefully I can get one with enough locally. Thanks for the help.