Shakey

The batteries are more likely to be in series-parralel banks. Series for voltage and parralel for capacity. So there will be a 'master' negative and a 'master' positive. Discoonect these first, doesnt matter which, thats only on cars where the chassis is negative and disconnecting the positive first will short the battery.

Get rid of the interconnecting links one by one.

The output of the UPS will most likely be connected into an automatic switching panel - just conectors with their sensing coils connected accross opposing circuits.

just find the panel it goes into it and isolate from there