We have added R10 polystyrene panels behind the pass through compartment doors. Heat taped the water line and enclosed the sewer line in a polystyrene box. We also have skirting on the way from http://www.rvskirting.com/
RV Skirting in Loveland CO. Curt Miller is the owner, he is a great person to work with!!
In addition to the heat tape I added a redundant heat source ( 40 watt bulb) on a thermostatically controlled plug in case the temps drop in the pass through......I have a wireless temperature sending unit in the pass through to watch the temps....hopefully I didn't miss anything..
I can post pictures later - if any one is interested??