This has caused me more headaches than damn near anything in a car. If you have a conventional battery, the kind where you can add fluid to...and have to do so on a regular basis, make sure the top of it is spotless. And check the connections. Not just where they are bolted to the frame or the starter etc. But the cable in the lug. Was chasing an electrical problem for a friend once and the problem was the ground cable was right under a very slow oil leak. The truck had an oem style cable that had the end just crimped on. The oil dripped on to the cable and ran into the lug. If the truck sat for more than a day or 2 the battery was dead. Was not so much that it created a drain but prevented it from getting fully charged. Fixed the leak, made a cable with the end soldered on and covered with shrink tubing, problem ceased to exist.....