Or, if you don't ever expect to get a battery
& the only thing you want to power is the fan, a less expensive solution would be a 120V to 12V power supply. It needs to provide enough current for the fan - if you give them a call at 800-521-0298 with your model number they can tell you what the maximum current would be. I believe they also sell a 120 to 12 volt converter made for their fans...
If you are also going to power lights
(particularly incandescent) than the RV converter is probably the beat way to go.