A Suburban thermostat is a combination unit. It has a thermostat and also a Hi-limit/Emergency Cut Off (ECO) part, connected together via a fusible link. If any of the parts in the assembly go bad, you replace the entire assembly.
Here is a pic of the ECO/Thermostat assemblies, with the fusible link visibly burned out between the ECO and Thermostat for the 120 volt AC electric.
The resettable hi-limit performs a different function than the thermostat, and you must have it..... so I would make sure you replace your Hi-limit/ECO thermostat assembly with an exact equivalent.