1979 Other Chevrolet Models Alternator won't charge the elec

I have a 1979 Chevy K20 that I have been working on. I recently replaced the alternator because the one that was in it went bad. About two weeks after I replaced it my truck refused to start so I replaced the battery. It worked for a few days then would only work after I jumped the battery. I had the battery tested and was told it was fine however the truck continues to not start. When I hook up a battery charger to it everything works however when I disconect the cables I have no taillights, break lights, windshield wipers etc. I replaced the alternator again with no change. Any suggestions as to where to go from here???