Remove the handle. Take a pair of slip-joint pliers and wrap
electrical tape or some other thick tape around the jaws. Use the pliers on the splines to try turning the mechanism. If nothing happens, the mechanism is broken and has to be replaced. If the window starts to open, then then knob is stripped and you need a new one.
You don't mention the year of your trailer. If it is older than 20 years and the mechanism is broken, you might want to remove the window, replace the mechanism, replace any deteriorating rubber seals and re-install the window with new Butyl tape.