[Marauder]

It cant be the battery alone.

The battery is used to start the vehicle. Yours starts fine obviously. Once the truck is running it will/can run off the voltage supplied by the alternator.

You need to buy a decent battery charger and charge it then see how it acts. It is NOT good for the alternator to try and charge a dead battery.

New battery wouldn't be a bad idea. Once a traditional battery (lead acid, not gel cell) is drawn down to almost nothing, it very rarely will charge back to full capacity.

(on a side note, it wouldnt be a bad idea to clean the battery terminals and reconnect to ensure good connection.)