Have you checked to make sure the thermostat is functioning properly? You may want to manually overide to check. If that checks out then I would pull your furnace out and give it a good cleaning. 30+ years of spiderwebs, dust bunnies and who knows what else could be a cause. Also, I read on another thread about the possibility of carbon monoxide leaks from a cracked tube on these Suburban models. Check out Andy's advice (Inland RV) on another thread. The thermocoupler seems to be operating ok if the pilot light is remaining on. Don't know what else to tell you, but others here will. Congrats on the new trailer and welcome to the forum. Good luck.