Just looking for some experience on this as I don't want to be doing evasive maneuvers in the middle of the night. Worst case we cram in the lower bed and put the top down. Lows look to be in the 20's this weekend up high in the sierra. Wouldn't mind closing down the campgrounds in Rock Creek and getting some fishing in. Snow will be there soon.[/QUOTE]
We have used our D4, which has the same BTU's, to below zero F with the top up, and even then it was on low to medium fan with thermostat set at 63. The side door footwell is draft prone and a rolled up towel across the bottom the back doors is a good idea to seal up that area.
You should be fine.
Couple of things to look at : Placement of the thermostat, heater duct outlet(s), and cabin air intake.
If the thermostat is placed near another heat source (fridge compressor vent, lighting, behind a blanket, etc.) , or above the heater outlet duct, then the thermostat will will cause heater to drop down before the rest of the cabin is comfortable.
Heater duct placement - this seems to be a balance between duct diameter for air speed and movement and duct placement. Ours is a single duct (right now) which comes out at floor level in the middle of the van on the driver side. This seems to provide a reasonable distribution of heat, but the front seat foot wells and the back door area is definitely cooler. For heat distribution into the penthouse bed area, we slide the bed platform forward to allow warm air that collects under the bed platform to rise up the back wall. A small fan strategically placed in the cabin can also help to more evenly distribute the warm around the cabin.
Cabin Air intake duct - make sure this intake is very clear of obstructions. There is a sensor in the heat exchange chamber which will shut the heater down if the air around the chamber becomes too hot.
Always a good idea to run the heater on high for a few minutes before shut down.
Fishing at Lake Sabrina was excellent from the dam and shore line a week and a half ago. have fun!!