The key is the working of the furnace for a period of time. That makes me think that it is going into lockout in which you must break the thermostat call for heat (see excerpt from manual below) or possibly the over temp switch. The furnace may not be actually be getting overtemp, the switch may by weak and breaking at normal operating temp. If the sail switch was bad I would expect the furnace not to heat at all. The propane should not come on until after the sail switch makes. If no intervening act the switch. most likely will not move so furnace will continue to run.
From suburban manual
"All DYNATRAIL direct ignition furnaces must have a flame sensing circuit through the control module that senses the presence of main burner flame. If proper flame sensing is not obtained or if the burner fails to light, the furnace must go into 100% lockout. When lockout occurs, the valve closes and will remain closed until the thermostat circuit is broken. Re-ignition will not occur automatically. If it could, it would not be 100% lockout.
While 100% lockout is a safety factor, it can prove to be frustrating for the RV owner who has experienced lockout problems. Not only can a lockout problem be difficult at times to diagnose, it will result in continuous operation of the furnace blower (that is, until the thermostat circuit is broken) and can drain the battery if the owner is not in the recreational vehicle.
