C3500 Something running battery down

1988 GMC C3500 - When the truck is parked for several days something is running the battery down. The charging system is ok. I can disconnect the battery cable at the end of the using the truck and go back and reconnect later and it work fine everytime . I have checked the brake light switch and disconnected all interior lights,radio,etc. I could put a battery disconnect switch in but I really would like to find out what is causing this. Any suggestion on how to track down the source of this problem would be appreciated.
Phil

Disconnect the negative cable and connect a test light between the cable and the battery. Then you need to start pulling fuses or disconnecting things (like the alt) until you find what is draining the system. The light will go out when you find the circuit that is the culprit.

//2000CAYukon