Again, thanks to all the experts who have chimed in here. Good advice from everyone.
Had a friend come over and look at things with / for me. Here's where I'm at:
Battery is not serviceable... headed out in a few minutes to get a new one. The 2 coach batteries are in good shape. With the new battery we should be able to make sure that everything battery related is good to go.
However, upon further inspection, we opened the electrical panel and noticed that the main 30-amp breaker has corrosion on the outside of it... on the top. We both scratched our heads and didn't know what to make of it. The only thing we can figure is that somehow it is corroded on the inside and that corrosion bled through to the outside of the breaker. In any event, it's not worth messing with, so I am going to replace that too.
It just seems strange to me that everything seemed to be working fine until I took it in to a shop. Now all of a sudden there are issues. Could just be coincidence... don't know. But the battery is definitely toasted, so it's getting replaced. The main breaker, showing corrosion like a battery terminal, is getting replaced. And the Isolator Controller, even if it's still good, is getting replaced because the online store I bought it from doesn't accept returns on electronic components.
As always, any other advice is more than welcome. I am not a mechanic, but I can figure stuff out. This internet thing is gonna catch on pretty soon but, in the meantime, I will use it to my advantage :p Cheers