Welcome aboard, and congrats!
To me, it sounds like your truck has a charging system issue. In order to better help, I'm gonna ask some questions:
* When you have the truck running, what does the ammeter say? If it's slightly charging, then you're probably okay generator/regulator wise. If not, clean up the armiture and the brushes on the generator and see if that helps.
* Is this truck stock (6v) or has it been converted to 12v? What voltage battery are you running?
* Is your battery old?
* How does the wiring look? Has anything been spliced in anywhere? Are there shorts that might be draining things?
I'm sure that someone else will pipe up with more questions, but this should be enough to get us started.
