Lots to think about from folks who have posted, thanks.
That last one has hot replacement batts too, but says this:
"Work though short blackouts & maintain a network connection / save files safely during longer blackouts: 1300va / 720w smart lcd battery backup uninterruptible power supply (ups) provides up to 64 minutes of runtime for an entry level pc system, and 10 min of half load runtime "
That is not very long! I like the idea of using an RV battery or two instead of what the unit comes with and get many hours. It is for when we lose power locally, but the phone company still keeps its going so there is internet to the house but no power.
If the outage is for longer, there is still the RV to use as a big UPS inverter/ battery bank and run extension cords to the house. But the first thing is not to lose the internet connection while in the middle of something important. Then rig up something longer term if required.