RV Repair: Suburban Heater Blows Cold, fifth wheel trailer, hunting camp

Question
Hello Shawn,
          I own a 1987 Canterbury Traveler 40' fifth wheel trailer. I currently utilize it as a hunting camp in upstate N.Y. My problem is as follows. I have a Suburban NT34S heater that was in place at the time of purchase (5 yrs ago) When first purchased, the heater would not blow hot air. I contacted a local (service man?) who changed the circuit board and cleaned out a mouse nest that was situated in the wheel house. The heater worked for a while and started to blow cold again. I then decided to change the thermostat and again the heater began to work and cycled fine for 2 winters. Now I am again experiencing the same problem. Example: I will arrive to camp. Turn on the gas. Turn thermostat to heat. Raise the thermostat temp. Blower always starts. Heat commences. Heater cycles. During the night blower will stay on and blow cold air. If the thermostat is turned down and reset the heat will again start but will eventually blow cold again. This past weekend it decided to cycle just fine at times. Also, I opened the access door to ensure that the unit was indeed firing via the pilot window and as long as the access door was open the unit threw heat. Once secured and closed the unit would not re-fire and blew cold again through the vents. Help Shawn!!! Thanks in advance for your assistance. Eddie

Answer
Hi Eddie. Limit switch. No doubt in my mind. With the door open, it dose not get hot enough to trip the "High Limit Switch". (that's the proper name for it.) I explained how to do this in past answers. 5 years is about the life span of them. If somehow I am wrong. (and I am not!) Ya right! Ck the sail switch. Good Luck.