This is an interesting subject.
I imagine the rules that apply for a brick-n-mortar house would also apply in a motor home. There are two primary contributors to a cold home.
#1 - Air Infiltration From Poor Window & Door Seals
PCs rank high in this category. Seal up the leaking air and you solved the worst of your heat loss and discomfort. Our PC has NO slideout so I cannot comment how well slideout seals do their job in extreme cold conditions, but I imagine the seals become less effective with age.
#2 - Radiated Cold
The front cab area, ceiling vents, and single pane RV glass are primary contributors.
Front Cab
As 2 Lucky mentioned, drape a thick quilt or unzip'd sleeping bag over the front seats and tuck it tight between the seat-to-wall sides and flat to the floor....seal it all very well. You will notice a really big difference between the temperature in the PC house verses the lower cab area. The air at "knee level" in the house will be much much warmer.
Roof Vents
Cover inside the roof vents with a product like
THIS.
Single Pane Glass
As mentioned by others, covering single pane RV glass with sun shade style foil wrapped bubble-pack material is effective as long as it seals tightly over the glass. I wonder how to seal it properly.
Phoenix does pretty well by RV standards with insulation in the floor, walls, and ceiling. We can only hope that the Phoenix insulation installer did a good job, leaving no air gaps. It's a good design as long as the workmanship is good.
I have noticed in our PC bathroom shower that when it is warm and humid in the bathroom in cold weather, I can see vertical cold-lines where the steel studs radiate the cold from the outside to the inside. There is accumulated humidity where the studs are. The only way around that is to use wood studs of which I'd rather not have.
Our 2007 2350 has thermal pane glass and no slideouts so our rig is smaller and better sealed than most PCs. So I think our situation is more ideal for "cold" camping. Having an electric site makes a very big difference, offering power for supplemental electric heat. I place a small ceramic heater near the quilt barrier facing rearward to neutralize any cold remnants from the cab area and entry door. When setup so, our furnace doesn't run much or not at all pending the outside temperature. And "Yes" we make certain the quilt won't fall on or close to the space heater.