You can get a single 12 volt switch for RVs online or from a local dealer.
If you have a thermostat on a wall outside the bathroom or a holding tank indicator or GFCI on a wall inside the bathroom, there probably is a hole in the horizontal top plate of the wall that you can fish new wires through. Take down the trim around the fan and if needed, ceiling light or AC grille to give you access into the ceiling cavity. Then use a wire coat hanger, piece of #14 wire or an electrician's fish tape to get new low voltage wire from the switch to the fan. Once in a while or for longer runs there may be an obstruction where you will need patience and numerous attempts to do it. If needed, you can enlarge the hole behind a ceiling mounted light for better access.
We have a Fan-Tastic ceiling fan in the bathroom (with remote) and normally have it running on low most of the time except for showering. A separate wall switch will allow you to have it running by itself or you could tie the light and fan together on the same switch.
After you've figured it out, you will know how to add wall switches for all of your ceiling lights if you want. Once in a while, you may need to go down and through a cabinet and back up to get new wiring installed. I've run lots of various wiring around inside the ceiling and up/down walls. Below are switches I added for the bedroom lights and a 3-way for the kitchen/living area ceiling lights using an existing hole in the top plate. It is really annoying when for a few extra $$ they could install wall switches at the factory.