the gate valve of the outlets of my gray and black on our TT are pretty close to where the 3" pipe (or 1-1/2" gray) drain pipes exit the "heated" enclosed underbelly. So... While I agree some drain pipe is technically exposed, the majority of the exposed drain pipes are downstream of the gate valves.
So with the liquid effluent temps being 40 degrees, the small amount of exposed "upstream" drain pipes should be a degree or two above freezing.
Of course with enough water flowing into the tanks, including some hot water, most weekenders will be fine. The troubles start while the unit sits for a week without any inflow.
Craig T. Bailey
2015 Jayflight 32BHDS
2007 Chevy 2500HD Crew 4x4 Z71