Just install a 12 V socket on the solar panel
side of the controller with a switch. Then insert an automotive USB charger into the socket. Then cut apart a USB cable and insert into usb port of charger. Then wire a 5 V muffin fan with less then 1 A current draw The controller would disconnect the fan once the sun went down, so no worries about running down your battery
A little bit convoluted I know. This issue is that once the battery
is fully charged, the "12V" solar panel
voltage will head up to its open circuit voltage of 18-20 V which would tend to incinerate a 12 V computer fan. I suppose you could just assume the battery
is full (good assumption for storage application) then use a resistor to bring the voltage down to a safe operating point for the fan. The resistor would need to be able to handle about 1/3 the power of the fan. When the battery was charging, the fan would mostly turn off, which isn't bad thing. The problem is that the fan would only run during full sunlight. With a little guess and check on the resistor value, you could probably get it to work OK. Now of course you have a resistor burning enough power to be a bit of a hazard in a constrained space.
I just leave the windows
open on the camper since I have the window covers that won't let rain in.
Maxxair 00-455000 Window/Maxx Louvered Vent by Maxxair