Furnace runs but blows cold

The first thing I typically do for an RV furnace or water heater failure to ignite is disconnect and reconnect the wiring and ignition/sensor connectors a few times at the ignitor board to see if the problem is as simple as some contact corrosion. The ignitor/sensor connection in particular can be a problem since the sensor mode is working with millivolts. If that solves the problem, then I suggest using an appropriate contact cleaner at the first opportunity. I've worked on at least 100+ RV furnace problems over the years, and only replaced a couple of sail switches. The rest with ignition problems have generally been failed igniter boards, mis-adjusted or cracked igniters, failed gas valves, etc. Reseating the connectors alone has worked a surprising number of times.

Maybe Doug Rainer will stop in and lend his professional expertise to the problem...