If you're going to put it in the shop, have them check the hot water line going to the front to see if it's in some sort of protective wrap, or if it's out there by itself where all of the cold are flowing underneath the DP is sucking the heat out of it? If it's not protected it, they need to insulate it with something such as the black foam pieces you use in a house to keep hot water pipes hot. Use a bunch of ty-raps on it.
How did you have the heater air direction set? Was it mostly on heat or on defrost? Granted you have to keep the window clear to safely see, but divert as much of the air to the floor or dash vents as possible.
And I'll be you have more fresh air coming in through the front end than you think you do. On my DP, there is no insulation under the dash - just the bulkhead itself. We have a very good heater, but I'm still thinking of using foam insulation under the front cowling to seal up any air leaks and provide more insulation.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.