Charging issue NEED HELP
 

So was wheeling last weekend and got back to where i was camping turned the truck off all was fine. Went to start the truck and my brake/charge lights were on. I was told by some guys this means your alt is bad. So i replaced it with a autozone one. Witch i have now from reading heard was a bad idea but had to use my original one as a core so no longer have it. Anyways i replaced the alt and light went off until the next day then came back on seemed to charge fine but came back on. Then the next day it seemed to die the batt was reading 8v when running but when i turned it off the batt jumped to 12v? makes no sense.Anyways i went ahead and replaced the batt to because it was 6 years old and figured it could be the issue. So put that in last night drove around lights were off no issues. Just started my truck this morning and the lights are back on? What could it be is the new alt bad? how can i test it on the truck and if not then what else could it be?

most auto parts stores have battery load testers and alternator testers, I know at least Orielly can test them on the car, I would start there first

How many volts are at the battery when the truck is running? Should be somewhere around 14V.

If you are going to have the battery tested, take the alternator too. They can check both.

If both are "good"...(please ask to see the results), look for a bad ground from the battery to RF inner fender or battery to engine block. Also check that your alternator harness connector is fully seated into the plug on the alternator (ask me how I know to check this ;) ). For fun, check the power line from the fuse box to battery for corrosion. Last would be to clean the battery terminals.

If battery is new and the alternator is new, my guess is that its a bad ground.

Yes...that 8V you got is telling you something. That 'something' is that your battery is actually being DISCHARGED somewhere while the truck is 'on'. I suspect a short somewhere.
I'd try seeing if there is continuity to ground from the positive cable...or even some resistance reading (not while attached to battery!). Then do the same with the alternator harness, etc etc., on the same lines as Angrybob said. good luck!