AC heat...running together!

Hello All, Our new TT, "Shadow Cruiser", has come up with a weird one....the air conditioner and heat run together! I was doing normal get ready stuff on a cold morning and decided to run the heater, when I set the thermostat to heat the AC and heat came on. Never happenend before! I thought maybe the AC relay was stuck...sealed unit...can't check, no continuity when power is off so not likely. I am pointing my finger at the thermostat now. I finally turned off the AC breaker to warm things up. The TT has a Dometic thermostat, digital.

Anyone have this before? I thank you in advance for help on this one!

Michael