Help with Coleman Electric Heat Strip using wall thermostat

You also need an extra wire between the thermostat and upper unit- I have used the standard thermostat, installing a separate electric/gas heat toggle switch. Not ideal in that you cannot run both at the same time, but I don't know if you can do that with any of the thermostats.
Another option is to simply add a separate furnace thermostat, and use the heat setting on the original for the heat strip.
Again though- this requires an extra wire between thermostat and upper unit.