Extra Battery in my T-160 FunFinder Shadow cruiser

[Izzak6th]

I acquired a FunFinder Shadow Cruiser T-160. There is a battery located under the Right
front bench seat in a black plastic vented box. Does anyone have any idea what this battery is used for. I found it while I was trying to determine why when I connect to external 120vAC the main battery on the hitch is discharged overnight (only when hooked up to external 120vAC) ??? I thought the converter was suppose to trickle charge the main battery and not discharge it when hooked up to 120vAC ???
Thank You for reading this and any help you can give me
Izzak Knott

PS the service manual that came with the RV is totally Worthless...

[shadow]

so you have two batteries? hook up to shore power and put a multi-meter on the output side of the charger and see if you getting any voltage should be in the range of 10-14.5 volts. I am assuming the previous owner added the other battery to improve boon docking time. check the charge voltage at each battery as well