According to the Dometic web site they provide an optional heat strip. The air conditioner in our 3400RL bedroom has a position on the switch that says "Heat," but it is inactive. See below quote.
From Dometic:
"
Optional Heat Strip
* Provides 5,600 BTU of heat?without any alterations to ducts or distribution box
* Heat strip is pre-wired in most Penguin models for quick and easy installation."