You might check the cranking voltage at the battery next time but it sure sounds like the battery is done. sometimes even if a good battery is discharged all the way it will kill it. Agreed, 15 yr life is unheard of! what I'd do is toss in another battery from one of your other Mopars & that would confirm that all is good in the 'hood ex the battery

live every 24 hour block of time like it's your last day on earth