I presume you are looking for a 12V
solution to use when shore power is not available. Otherwise you could use the fan in your AC.
The furnace fan only circulates inside air. It is noisy, however.
There are some nice small fans that run off USB. If your trailer has USB outlets, you could use them. If it has cigarette lighter sockets, you could get 12V
to USB adapters.
Fantastic Vent (now part of Dometic?) makes a fan like a small window fan that runs from 12 volts using a cigarette lighter plug. I have one on my boat. It does a great job.