Many ways to solve "cold fuel" problems.......in the tank... Additives, such as are available at any truck stop. And yes...even that "other" fuel. Doesn't take much of it, depending on your tank size, a soda can or two, usually does the trick.
BTW, while the Webasto will warm the engine and heat exchangers and water heater, not sure how it will warm the fuel....unless one wraps the fuel tank. That would take a lot of copper and a lot of warming, seems to me.
FWIW. Where's Fast Fred?