Stop at any of the big truck stops. And look for the truckers section.
Usually it's tucked in the back where the truckers showers entrance would be. Flying J has a fairly good truckers item section too.
They have a plethora of nice, small, quiet, but real powerful 12v Fans.
Plugs into your cig lighter, or the 12v outlet next to your antenna booster switch. And I have even used a portable jump starter to plug the fan in so you can place it anywhere.
And I might add a lot of other real useful RV items you don't see in the stores. I buy all my stuff for the RV from the truck stops.