@Serpico
Well, I'd expect for much of vanlife, external power isn't even an option, presuming you're parking somewhere where shore/grid power is inaccessible, especially if you're a 24/7/365er.

Beyond that, I have no idea.

I have tried looking up the draw of particular block heaters draw to no success, but abstract/unreferenced/unspecified websites are claiming a range between 400 to 1500Wph (that feels like a pretty broad margin of error) for a 110v unit (which I believe my current one to be).

Unfortunately, I don't have the means to find out the precise draw of my current heater, nor had any success trying to look up its specs or the specs for the heater on the theoretical van I'm designing. (And I haven't locked in my onboard battery size either.)

This is not something I claim even the faintest degree of expertise in, which is why I'm asking. (If I knew a lot about this sort of thing, I wouldn't be asking, I'd be telling.)