You're looking for an RV "converter" not inverter. It converts 120v AC to 12v DC for your 12v fans. Most have another wire that's set up to charge a battery.

The cheapest way is to just get a little 120v fan.

A battery tender puts out very little current, it won't keep up with any accessories. It's only meant to cover battery self discharge and a clock and radio presets.

A real battery charger will work, you can use it with the fans on, but only part time. I'm thinking the old style with a regular transformer inside, they're pretty heavy. New style switched mode might shut off when you turn something on, they're "smart" and panic easily.

An inverter will let you make 120v from a 12v battery. Wrong direction.