Bad Z fuse (fuse N3) will keep the Aux positive side isolated from the main battery and the alternator. The aux battery will discharge with no way to be recharged by the alternator. It runs a bunch of the electronics so those things will start getting glitchy. Your mention of it working fine for 4 days then going glitchy seems to fit this diagnosis from the other side of the internet. N3 is also the fuse that will blow if someone shorts the positive cable to ground accidentally when swapping batteries.
Nothing in your original description of your problem sounded like an engine problem. Maybe it had a problem, maybe not. Either way, you got a fresh new one for free.
Wow, when I saw the word "Gremlin" in the title I think of the pumpkin orange '75 Gremlin my next door neighbor bought around the same time I bought my '75 J-10 pickup; in the same week.
The Gremlin was a quirky little bastard but it was fun to drive.
