RE: Battery dropping
Mike, do this to make sure your change to the aux wiring is working the way
you wanted.
First, does the radio work with the key out? It should.
Either way, turn on the radio.  Now disconnect the ground strap on the aux
battery.  The radio should go off.

I suggest this simple test because when I rewired all aux items to the new
aux battery I (actually the guy helping me) had to make several other
changes on the back of the fuse block to get everything moved over.

If all that checks out then again check the charging voltage AT the starting
battery, not at some remote point.  That will tell you if the alternator is
giving you what it should.

Based on what you've been saying I still suspect the battery itself.