What I'm saying is, use a water heater to power the thermosiphon loop and run the coolant through where the water usually is. Use a dual power unit so it can be run on shore or LP. Then that's your low level "keep warm" heater and you can fire up the webasto for on-demand use. With the water heater in a storage bay and the heater cores at floor level you have enough height to do thermosiphon so the only energy cost is the burner or heating element. 6Kbtu is about the most you can expect from a water tank heating element at 220V.Jim
This is one of those times when you've got to look around and see what's being commonly installed, both in bus conversions and in commercial motor homes, and ask if maybe they're on to something. I'm not knocking the thermosiphon idea in principle, just in application. I enjoy doing something just to see if it works as much as the next guy, but in this situation there already will be in place a few redundant systems - heat generated by the engine, the Webasto, electric heat pumps, and an LP furnace. Is there need for yet another? is there enough advantage to thermosiphon to add yet another?