Where is the aux battery located on a 1994 Coach House 192TB

I bought this vehicle back in March. The vehicle chassis is Dodge 350. I have used it 4 to 5 times for overnight trips. We went on an extended 4 night trip last week and on day 2 I noticed the lights in the bed area were getting dim. By the time we arrived home (last night), we had no lights and the control panel was not working. I suspect the auxilliary battery has gone bad. I searched under the two beds and did not find where the battery was located.

I've looked under the hood, but not under the "doghouse", yet.

Any suggestions on where else to look?

If the issue is the battery, will hooking it up to a charger solve the problem, or will I need to buy a new one?

I'm not mechanically inclined, so your suggestions are greatly appreciated.