When installed in the RV, does the gas valve click and Ignitor click? Or it only does when you remove the furnace for bench test? You need a voltage measurement AT the Blower motor 12 volt wire at the motor pigtail. When installed in an RV, there is more CASE pressure than when on the bench. IF there is a drop in voltage, combined with the increase in case pressure, that may drop the motor RPM's just enough to not fully engage the sail switch. Also, long shot, a bad motor and cannot overcome that case pressure to keep the RPM's at speed. Doug