The reflectex in the penthouse makes a huge difference on inside temps. We regularly camp in the winter time in temps down to the single digits. I have had my share of frozen pipes but I have come up with a couple of things to help prevent it.
1) Always keep the furnace on at night at least on the lowest setting.
2) Keep the door for the water tank/pump area open and the under sink cabinet cracked a bit.
(In your case you don't have these cabinets so it shouldn't be an issue)
3) Add insulation to every nook and cranny in the van. In the doors, walls etc. There is a huge voice space above and behind the taillights!!!
4) Turn the control on the dash to off. Leaving it on vent will allow outside cold air to come in.
5) I re-routed the water lines in my EB-50 to all be interior, not in the walls. The factory routing was asinine to say the least.