Our 2011 327DS, as far as I can see, doesn't have any warm air from the furnace going into the plumbing compartment. Two of the insulated pipes run through the compartment containing the fresh water tank and water heater but I've never seen any venting into the compartment. The waste tanks are in a separate compartment and there's no heat there other than the arctic pack heaters.
I added a 300W home gutter heater strip to the plumbing compartment. I zip tied it to all of the water lines and have a double wrap around the water filter. It plugs into a thermostatic control that turns it on only when it's needed. I also have a cutoff switch inside the coach to completely shut it off. The outlet is connected to the inverter I had installed when I bought the unit which gives us 110VAC for the heater and fridge while we're driving. Overnights during cold weather are always done at a campground where we have shore power available. The only remaining problem I have is that there's no valves in the compartment for the low point drains, just plugs on the end of the pipes underneath the compartment. I'm expecting to install valves in the compartment before we start on our next winter trip.
Last Winter, I had no problems when we stayed at a campground when the overnight low hit 18F. My decision to run on internal water and refill before leaving was a wise choice. Another camper was using water from the hydrant late in the evening and the hydrant froze and burst a pipe underground. When things warmed up in the morning, the spray from the break made a great show. The campground owner stated that the camper would be responsible for the repairs - aorund $1000! I didn't believe this until last night when I researched frost free potable water hydrants on the web. It appears that they can cost over $650 apiece and that doesn't include the labor to install them.