I do know Advanced RV uses reflective insulation which does add some R-factor. However, in general, at most, it might be a few inches, just because there isn't that much room in a van. Even with insulation, the single-pane windows on the windshield and driver/passenger doors are an issue. The windshield can be helped by a cover on the outside and inside, but the cab doors are difficult to insulate. The back doors are also an issue, not just with windows, but with warm air getting out.
I have wondered about aerogel insulation types, like Thermablok. Of course, these would be more expensive, but for a "B", more bang for the buck is required.
As for heaters, I wish the furnace makers here in the US would take a clue from Espar or Truma, and go gas/electric. Bigger rigs like "A"s have Cheap Heat as an option. This way, regardless of energy consumed, it would help things. On really cold days, both could be used in tandem.
