Not familiar with Coachmen units, but the fan is 120volt, correct? A portable fan? A slowdown of the fan speed indicates to me the switch turns on something 120volt with a high draw, possibly the electric water heater? Do you have an electrical panel with any labeled circuit breakers? If one is labeled for the water heater, turn it off and see if the fan goes off. If it does, the fan is on the same circuit as the water heater, and that is what the switch controls.
Other than this, just start a patient, orderly check of what is on each circuit in the panel.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired