high current UPS ish design
A low-noise 80A battery charger should be able to deal with this. It's the same kind of issue that folks confront at trade shows to keep car batteries alive while they're powering all their gizmos on the show floor.

Example: http://soundsgreatusa.com/cascade-aps-90-amp-1200-watts-pro-grade-power-supply-dual-volt-marine-ready/

Another: https://www.powerstream.com/charger-low-noise-high-power.htm

The power supply would connect to an automatic transfer switch, which would be part of the genset or a separate item.