Run the coolant lines, its free hot water. If you have an Espar under the hood why aren't you using it for heat? Forget air heaters and add a couple of radiators. Diesel cookers have a mixed review at best.
I have been really nervous about running coolant lines back into the habitation area, but I don't have the cargo capacity to pump glycol around the rig, leaving engine coolant the best option. When folks who have the real experience such as yourself comment, it seems using the engine coolant is a well accepted practice. Using a free byproduct from the engine to accomplish so much makes sense. I could bypass the engine, then run a coolant line from the existing D5 through the water heater then through a 1,700 wall exchanger (7 watt DC), before returning to the D5. The in-dash exchanger would be in the circuit, but I doubt it's DC power efficient enough to use a whole lot, but some. It has a nice REST feature also to harvest heat 30 minutes from shut down. Nearly eliminating the need to burn LP for the water heater is huge!!
If I did this, I could get an Espar D2 Airtronic and run that when it's colder. Combined, I'd have 3,700 watts in the D5 + D2, less derating for altitude of say 25%, so 2,775 watts usable, worst case, which is where I'd be at with a new D4. Then I'd have the dash fan exchanger if necessary.
Mercedez modified the Espar D5 by using a MB ECU controller and MB water pump. I got to figure if the water pump will handle this.
Does this sound reasonable?