replace the cable and terminal connector... i dont know why, but having a bad cable or connector to the battery just saps power. i dont know where the power goes, it just goes. could very well be a bad alternator, just run it to your local autoparts chain store and have the charging system checked. or dont do any of this and just disconnect the negitive when it sits, then its just up to the battery to hold its charge while it sits. imho, not a great idea to let any rig sit... it completely wrecks havoc on so much and the batteries charge is the least of it. at a minimum it should be started and ran for atleast a little while at operating temp, even better if you can take it for a short drive.