If the UPS is only powering a computer then it should be possible to replace it with a very simple battery charger and a 120 V battery because most (all?) moder

If the UPS is only powering a computer then it should be possible to replace it with a very simple battery charger and a 120 V battery because most (all?) modern computer power supplies are direct online switched mode and can operate on any input between 100 and 240 V. The PSU can accept DC because the rectifier is on the high voltage side and there is no 50 or 60 Hz transformer. This eliminates the inverter and removes the need for any bypass or synchronization circuitry. Of course you need to check carefully that your particular PSU can operate like this before trying it. Alternatively surely there should be a market for PSUs that accept 12 V or 24 V input so that they could be driven directly from single car or truck batteries. Or the equivalent lithium ion or whatever else is the best available technology of the moment of course.