Adding on here to D5 info that may be helpful to folks:
After 5 years of awesome service with our system and many winter trips it failed. I took it to Thermo-King (typically only Espar service org around) and they worked on for a day and did basically nothing but replace connectors hoping that would do it. Probably should have driven it to Rixen's in OR right then.
I found it had coolant in the control board -not good. I ordered a replacement furnace from Butler in the UK - good prices. However they didn't tell me that the newer system also had to have a replacement high altitude kit (same part number, but it indeed has changed over the years). WIRING TOO. You must get a new harness and rewire. The HA kit does its signaling to the unit over different wires.
The new unit was not plug-and-play - it was larger and I had to rework the brackets and hoses. The unit worked for a while but intermittent failures and then total failure. Upon research it turns out the new unit (D5WSC) has an internal fuel pump and an internal coolant pump. There is no direct replacement, you must get this newer unit (according to Butler).
I wish they had told me about the pumps! Then again maybe I should have realized why the new unit was larger. Two coolant pumps may not be a big deal but two fuel pumps can certainly screw up the mixture. Removing the external fuel pump and repiping was easy, but removing the coolant pump and rehosing was a messy pain. The new configuration also required priming of the system which took some creative use pressurizing the coolant tank.
I think it works now, but hopefully this helps someone.