Replacing thermostat for gas furnace only

Have a 2003 Fourwinds with the old style wall thermostat with the hard on/off switch and temp adjustment below with just bars (no numbers). On several occasions, the furnace will start and heat up camper but won't continue to keep comfortable. Would have to get up and bump it on and sometimes it would stay on then not work again an hour later (sleepless nights). Any suggestions on replacing it with a more modern unit? This only works the propane furnace ( have a separate ceiling a/c unit with manual knob adjustment). Thanks!