I Have an 2017 Octane 222, i know its a toy hauler (different forum), I have the same problem. Fan runs for 1-2 seconds thats it, I did replace the analog t/stat with a digital, I Was going to do it anyways.
I had a the Hot Water Tank rupture/froze because of lack of winterizing (Dealers Incompetence), Furnace sits next to Hot Water tank, Im thinking now, Furnace got wet.
No way to prove it, the other cleve It is that it sits under the cabinet holding the Fridge.
I concur with Aljo about adding an outside access panel. Its hard to get anyone to come look at it, but I leaning toward the control board getting wet.