Whats a good voltmeter to get?

Amps on the positive side show how much your charging system is having to put out to compensate losses to the batteries. So right after you start the truck, it should be higher and gradually drop closer to zero as the alternator charges the batteries. Once you start using accessories like lights, A/C, etc, the gauge will move up a little with each thing and that shows how much your alternator is "putting back into the system" for lack of a better word.

Amps on the negative side shows how many amps are being drawn off the batteries too. Problem is, the alternator isn't fighting the battle for you anymore. Could be broken or loose wires, a belt, fuse, things like that. In your case, I'd agree with the other posters and say you've likely got a bad alternator.

If you don't do work yourself, the shop can load test your batteries and rule them out. Then that will pretty much tell the story.

 

My Pete has factory amp gauge and I added a volt gauge. Every time i’ve ever started it. The amps immediaty go to +30 and than settle down to 0 after a minute or two. + positive means the system is charging. If you are showing - negative than the batterys are discharging for some reason. Put a meter on the alternator to check and make sure everything is working. Amp gauges are sensitive to voltage spikes and prone to go bad and they either won’t move off of 0 or false read on - side
You show over 12 volts in the pic 12-14v range is good. Its normal for when Lights and stuff on to be down around 12v. Than with everything off to be up around 14v.
Leads me to believe a bad battery or batteries causing discharge.

 

Your alternator is not charging. Your amps are below zero

 

If everything is off, then I have to agree with @SmallPackage. There's probably a short in one of your batteries, and it will gradually discharge all of them. If you start the engine, and that helps, then the alternator is probably charging. I would honestly start with checking the batteries first at this point.

 

Whoops! I was thinking that was with everything on.
If that is with everthing off than when you turn everything on it could drop to 10.
Check alternator. It may still be spitting out 12v but no amps.
Test and Charge the batteries separately. Than hook them back up and see if discharge again. Than you know alt is bad for sure before you take it off engine.

 

If a battery is bad it could do the same thing but you have to isolate it. That is why you have to test and charge them seperately. One bad one can bring all of them down and make it impossible for the alt to charge them. A good charger can charge them all together but there is no way to tell which one is the bad one.