Something is draining the battery overnight?

i recently been trying to start my truck and it doesn't have any juice left at all not even to turn on dome light. i had the battery replaced and alternator and will doing so i noticed the belt tensioner was bad so i replaced that too. once i jump it i have to give it gas for a couple of minutes if i don't it will turn off. after that it will run and start up all day fine unless left off for more then 4 hour then same problem. i have a stereo system in it but i have disconnected it and still have same results. i make sure every light is off and button every night but same problem how can i find what is draining the battery at night.