Welcome to the site..

1 Is there any lights on on the dash?
2 do you have a Volt meter?
3 have the cables to the battery ever been changed?
4 is there a ground wire between the engine and the cab, if so clean it
5 how do the wires look near the alt. are they hard as a rock or flexible?
6 In the manual section, you will find the UTI training manuals. In there is an electrical section that will help you. With a volt meter you will be able to find the problem. Please take time to skim through it. It will show you how to check for voltage drops. If you don't have one go out and get a half way descent digital one.
Charging voltage at the alt. 14.2VDC voltage at the battery will change depending on the charge of the battery. The more discharged it is the lower the voltage is at the battery. But the alt. will be 13V to 14V
7 are the light nice and bright or are they like a couple of candles in a can?