Check that copper bulb and tubing running to it for wear points. It is filled with a gas and if it leaks
, its ruined. Sounds like its no longer sensing the temp change or the gas valve is not working. They were used in tent trailers and truck campers from that time frame and should be showing up on Kijiji or craigslist for cheap. Parts from a Coleman gravity furnace
MAY also work but not sure.