Adam, we had kind of a similar issue with ours. My Webasto would work fine on 12V heat, but would not fire and run on 12V water heater. It turned out we had a bad Diode on the relay panel. If you gain access to your relay board check to see if it has a Diode between your Webasto block heater & 12V heat. The Diode we had originally looked OK, but when I touched it, it fell apart. I found this problem after we replaced the high temp sensor, and quite a few other components on the Webasto unit itself. Well at least I have more spares.
Steve & Rhonda Bennett
Dana Point, CA
92 Marathon XL40
2012 Jeep Rubicon Unlimited
Owner: California Coach Company, LLC