Help with Coleman Electric Heat Strip using wall thermostat

You can use the approaches listed above (adding a switch or a second thermostat), but Coleman makes a thermostat just for this application: Model 6536A3351
It has "gas heat" plus "electric heat" switch settings.
As stated above, you still need the extra wire from the thermostat to the A/C unit. That's sometimes easy to do, and sometimes not. Thankfully, my RV had an extra wire in the existing thermostat cable.