When I had my PDI on my 2011 Georgetown 327DS, I asked the same question. The answer was: this is to prevent you from storing stuff in this compartment. Of course, it also prevents you from changing the valves so winterize and makes it difficult to extract the winterizing hose. It's also impossible to change the water filter with the cardboard cover in place.
The compartment door has styrafoam insulation in it and the R factor of the thin cardboard is meaningless when compared to the door insulation. The compartment plastic box, where there's no bubble insulation is the greatest heat loss area for the compartment.
I removed mine and use the compartment to store the filter wrench and a few spare filter cartridges. It also makes it easy to visually inspect the tank when filling it so I can avoid the dreaded "siphon overflow drains water tank" problem.