My 2003 Chevy 2500 duramax has a battery draw somewhere that I cannot figure. I can use it for a few days in a row, then it sits overnight and batteries are dead. Have replaced them 4 times in 18 mos. Chev dealer thought it was a relay(?) that activated the glow plugs-replaced it-Nope. I replaced the original alternator-Nope. Local mechanic replaced a relay that controlled exterior lights, unhooked brake controller-no luck. I don't think this is just a small current draw, it seems to discharge pretty fast when it happens. It's useless to take on a trip with this problem. Any suggestions, previous experience like this?