Sounds like you're having trouble getting and maintaining enough current to the starter (that "clicking" you hear is relays being effected by Low Power) . Winter is coming, it's gonna take a 100% system to start if you're in the real Cold. Get it fixed now
So this would be about the battery Cables, Connectors, and the Batteries themselves (maybe even the starter solenoid if
your '01 has one on the pass side inner well ~ some do / some don't
The cables can suffer in two ways, the Ends can be corroded and dirty (scrape, clean, grease, and tighten them all - both sides
- and the battery terminals too). This is a Free fix so I start there. Next the cables the can be corroded internally, the copper inside gets green and can't pass high current (called a high resistance cable).
Do what you can, if it all looks good but still giving you trouble, have the batteries tested. Charge the batteries back up Fully, then have them Load Tested
, Individually, at a High Rate (not those dinky 100A tester the Parts Stores use on 6cly gassers, you need @250/300A load to test ours).
That would be the Best Start to solving this...