keeping batteries charged

We usually don't boondock, but will be spending 3 days in Death Valley in Nov, want to be able to run my 3 fantastic fans 24/7 (have an old dog who doesn't do heat well) other than the fans the only electrical draw would be to the converter and lights which are led, everything else fridge and water heater will be on propane, I realize there is a draw from sensors and to keep the dsi working, so I'm wondering in an 800 W generator running for 4-6 hours a day would keep the batteries up. I really don't want an expensive generator as I hope that we really won't use it other than this trip. Thanks