Ford e450 secondary battery questions

I have a 2008 Ford e450 shuttle bus that I have been having battery/electrical issues with. I changed the battery under the hood 6 months ago, but have had it be dead multiple times when I have gone start it. Twice now the battery could not be jumped, I brought it into an auto shop and had it charged, it was completely dead. Today while pulling out the battery I saw a sticker under my dash showing the location of the second battery. I did not know the vehicle had a second battery, and I am having a hard time accessing it. Could this be drawing power from my primary battery? I believe it may have been powering the wheelchair lift that used to be in the vehicle but I have since taken out, but I’m not sure. Do I need to replace that battery and/or put an isolator on it?

In addition to the battery dying, I have had other electrical problems like the side door occasionally stop working, and the driver A/C has been out. I do not know if those things are related.

All advice/tips are welcome, thank you.