Miles- in the morning, put a voltmeter on the battery. A fully charged battery at rest will show 12.65 volts. 12 volts is discharged. Look at the cables for open areas in insulation, for copper showing- esp. for corrosion visible in the cable. Besides having clean terminal-post connections ON the battery, also look for dirty corrosion crud on top. That dirty accumulation is conductive. IF you see that crud on top, put the + Voltmeter lead on Batt. + post, then probe around the batt. top w/ the voltmeter -- neg. lead. You'll likely see voltage on the meter. When that dirt crud reaches both batt. posts- it forms a conductive path. This leads to the battery slowly discharging itself!
If the battery cables show wear, corrosion, open insulation, consider replacing as needed. I've replaced worn Negative cables before ( and added an extra body or chassis ground wire at the same time)-- and seen Immediate improvements in battery and charging system condition.
It's easy to overlook the simple stuff w/ the battery and cables, but those little thinks really matter. Cold weather is coming, Git 'Er Done, Podner!
Tracy G