I'm beginning to think if they included everything we wanted to know in the Owner's Manual, the book would be 3000 pages thick!
I am new to Airstream and was pleasantly surprised this summer to discover that the condensate drain for the A/C runs through a drain line to exit under the coach, not run down the side like most others. This is a nice touch, Thank You Airstream. I also realize that this drain tube could get blocked by mud daubers or something similar, so it should be kept in mind that if water runs off the roof when running the A/C, this should be the 1st place to look.
However in heat pump mode, you should expect to see condensation run off the roof. A heat pump is simply an air conditioner running in reverse. The condenser and evaporator swap roles and moisture condenses on the outside unit. I'm sure the engineers decided not to run this through a drain line because in freezing weather, the line could freeze up. The simple solution is to let the condensate run off the roof.
As the temperature drops, the outside unit will actually frost over. In my Comfort Control Manual is says that when the exterior temperature drops to 42°F, the unit will go into defrost mode for 5 minutes after 25 minutes of operation. At that time it stops the blower and essentially goes into A/C mode to heat up everything outside. At 30°F, it switches off the Heat Pump and turns on the furnace.