1st quest what temp were you trying to heat to? We find that 68 degrees is very comfortable. with that last January we were hosting at the coast. We were very comfortable in our 32' three season 5er, at 20 degrees using the Cheap Heat system , which is only 5,000 watts of heat or 17,600 BTU. Our gas furnace is rated a 30,000 BTU input, or 24,060 BTU output. The cheap heat system didn't run anywhere near constantly.
You either have a huge cold air leak, a very weak furnace, or trying to heat to about 75 to 80 degrees.