Starter problems

[Dead Parrot]

Possible the starter brushes are near End of Life. Intermittent contact is one of the symptoms. Same for a near EOL solenoid.

give the battery cables a good look over. Cable internals can be bad while the cover looks great.

Don't overlook the possibility that one or both batteries are bad out of the box. Rare but it does happen. Disconnect one of the cables so the batteries are not in parallel. Let it sit for a few days then check with voltmeter. If one is much lower then the other, suspect bad battery.

Any chance a stray circuit is staying on and slowly draining things?

Is the brace still on the starter? Often skipped during starter replacement. On the 6.2, it is needed to keep the starter properly lined up due to the extra torque needed to start.