A typical 12v RV fan will draw between 400-500 ma. So you can plug it into a cig type outlet, most 12v RV fans already come with that type plug. If you use the upper 500ma (1/2 amp) draw for your calculation, the fan will use 1 AH (amp hour) of power every 2 hours.
If you have a typical single battery that came with the camper it is more than likely about 80-90 AH, of which 40-45 AH is usable before you need to re-charge the battery.
How long you can run the fan depends on how much power you draw from all other sources, lights, furnace, CO detector etc.
2007 Surveyor SV230 - 200 Watts Solar/MPPT Controller - 230 AH Battery Bank - 600 watt PSW Inverter - (2) 2000 watt Inverter Generators - LED Lighting - Boon Docking 99% of the time.
2009 F150 - 5.4 Litre - 6 speed - Tow Package