The official Jeep (FCA) method for changing the AUX battery is to remove the right fender completely (!) and then access the battery tray from below.Steph, I noticed above that you said you have to remove the fender to get to the second battery. Not sure why this is the case on your Jeep, but if you remove the main battery, the second one comes out quite easily after that.
Another method is to completely remove the fuse box (including the 3 or 4 very large connectors underneath), then push those large cables and finally access the AUX battery from the top. Several members explain these two methods here and on youtube.
All this is of course for a left hand drive (steering wheel)