The pumps start to work poorly getting below 50, and just about not at all below 4.
In simple terms they work the opposite of the air conditioning cooling mode. This video gives a good discretion of how a heat pump works. AFAIK, RV AC units with pumps do not have defrost cycle.
When I ordered the Cyclone I ordered it with 2 heat pumps. After the order was placed the Dometic plant in Mexico burned down - so the trailer was built without the pumps.
The dealer installed 2 heat strips for $105 each. Less eh an hour each with a t-stat change.
The heat strips will not heat up the trailer at a rate you can "feel". But, once on and running, especially overnight, they do a good job of keeping the trailer comfortable when heat is needed. I do supplement this heat with a space heater, especially since the garage is uninsulated space. They work best with the fan on constant and the speed set to low.
For the cost and performance difference I would go with heat strips.