Alternator and wiring?

My truck keeps going dead like the alternator is going bad. I've bought a new alternator and a new battery and replaced them both. But the problem still exist. The truck will run, depending on how much charge the battery has but the battery still seems to run down with a brand new battery and alternator on the truck. I've tested them both and both have tested good. So why is it that the truck keeps going dead if both have been replaced and test to their fullest capability?