What it the battery's voltage? Is there a way to hook an external battery to it to allow longer run-time?
I'd like to use it in a motorhome and have the 12 volt batteries as the back-up power source. That would allow me to run the motorhome off AC when parked and off the UPS when driving.
Yes with a constraint.
Make sure you do not exceed the power rating of the device 1080VA and it will provide you power for a limited period of time till the battery runs out. The battery in the power device is not a 12V battery.
Depending on your load, the time could vary from 15 minutes to a few hours.