You should check the label on the fan to determine the amperage the fan draws and select the best wire size for that amperage. Most RV fans draw little current so you can use "stranded" 14 AWG or even 16 AWG wire. Don't use solid wire. I use 16 AWG for most of my 12 Vdc wiring. Wire size is all about the amperage flowing over the wire; wire to small for the amperage will heat up and possibly catch fire (something that should be avoided as it can ruin your day). Here's a link
to assist you in selecting the proper wire size.