This is exactly the same problem I found about a year ago... the terminal wingnut had worked loose... primarily I believe due to the lack of lock washers and also due to the "stack" of ring terminals.

In reality one should not have such a stack of ring terminals, there should only be at most, two connections... one large cable for the starter and a smaller albeit still heavy cable to a distribution panel or terminal strip.

I had very similar readings as you... the charger appeared to be working, the voltage monitors showed good, but when attempting to start, the system could not deliver a large jolt of current. (actually acted intermittent) My first guess was that the battery was going dead... but when I got in there and dug around, I noticed the loose wingnut and proceeded to fix it with a lock washer and reduction in the stack of ring terminals.

Have not had an issue since.