Furnace Fan won't turn off, 2004 Alpha 40'

I am a new motorhome owner (8/2016). My furnace's blower would not turn off with the thermostat (blowing air, no heat) so I accessed the main switch through the outside panel to turn it off.

Is there a filter somewhere that could be clogged and causing the safety relay to turn off the heat and keep the fan blowing? I've poured over the book and opened every panel I can find and am confounded.

Thanks for any input.