If your battery is fully charged you may be OK. A sears group 27 is rated at 105 amps - 20 hour rating. So about 5 amps/hr for 20 hours. The higher the current draw the faster the battery is depleted. If it is a variable speed fan then the max current draw should be at the highest speed. I would run only one fan at a time.
Remember there are other items drawing DC power from the battery. Refrigerator, CO detector, TV amplifier.
Our bedroom ceiling fan has a thermostat so I set it and it turns off at the temperature we want it to. We also set it to pull air out of the RV.
I installed a second battery in our rig - both are in a battery box designed for 2- 6 volt batteries. We only dry camp twice a year at the racetrack - but having the second battery gives me some piece of mind.
