Random fan under bed with vent out of the blue 87' Rockwood

This is a heater that gets heat from your engine coolant. The engine coolant probably also goes to your water heater to make hot water while in transit. The under bed heater is most likely an Evans Motoraid and there is probably a switch on the dash to operate it. The switch probably has 3 positions-- off, low, high -- for fan speeds. These heaters do a great job heating the rear of the coach while in transit.

Have fun-- Mick.