As Titus stated would also check the fan/alternator belt for proper tension,being an older truck isnt to many circuits that would drain the battery overnight,just lights(if it has them) and horn circuit.
Also check the cable from the alternator to the battery for corrosion and tighness of connections, and do a voltage drop on it,also check for corrosion at the battery terminals and do a voltage drop there also on both positive and negative cables. Check to make sure cables are not corroded internally,have seen cables that look fine on the outside but were full of green junk on the inside of the cable from copper corrosion.