Having some issues here and just wanted to see what some other people think.

My trucks voltage is fine when nothing is on (stays at the M of normal) but if you turn the lights on and defroster on it will really drop. I had them and the wipers going the other day and turned the turn signal on and thought it was gunna die. So I took it to have it checked real quick, batteries are good at 12 volts not running started it up the guy went to test the alt but couldn't because when he put the clamp on the positive cable it was reading that the truck was idling at 2800rpm (wasn't, it was at 600 or 700 like normal). so he flipped threw and it said the alt was charging at 40amps only but I don't know if thats true or not.

Just wanted to see what you guys thought before i spend the bucks. The batteries checked out good so I would think its the alt and plus like a friend mentioned the batteries are really just to start the truck and then the alt takes over from there. I'm 99% sure its the alt, just need the last 1%

 

When the truck has been running for a little while put your hand on the alternator. If your skin develops little red bubbles from the extreme heat, your alternator is working and you have a short somewhere in the system. If the alternator is not that hot and the voltage is fluctuating, it is likely defective.

 

so i should press firmly and hold it there for at least 45 seconds right

 

I would do a voltage drop test first.
by your discription of events it sounds like your batteries are drained. batteries must be fully charge befor you do the voltage drop test.If your batteries show good then have the alt. checked

 

ok your alt is bad, 40 amps way too low, that stock alt should be putting out at least 100 amps at 1000 rpm. it don't matter how the batteries are, even if they are dead. that truck after started will run without batteries. and if alt is good after you run engine 10 minutes then shut off the batteries should read 13 volts at least, if not you alt is bad. I am a retiered auto electrician, and industrial machinery,

 

Another quick question. Went out and checked the batteries after the truck has been sitting for a couple hours now. Passenger side 13 volts driver side dead. Think i found my problem.

 

You probably have found your problem, I would change both at the same time to avoid other problems. I had a loose connector on one of my batteries, not sure for how long, I mean really loose, not making contact at all. When I found this problem I cleaned the terminal of all the pitting and corrosion and reconnected it. After another 2 months I replaced both batteries and the alternator. So, after getting them both replaced I would still check the alternator. The simplest way is to check the voltage at idle, should be between 13.7 and 14.8 volts DC. If below the alternator is shot, if above that the alternator is shot.