I was worried for a moment that you were thinking of running an exhaust into your car

But, this is not as bonkers idea as it sounds, the water not the exhaust gasses. Could be some mileage in it somewhere, although it would need a circulation pump like you'd use on a central heating system I think, rather than the type of pumps we use.
What about copper tube, low diameter like gas pipe? Wound around the down pipes, with anti freeze added as well. Then up into the van and connected into plastic under floor heating pipe, that's flexible sort of and tough to break, will radiate the heat very well in your tank.
My old man designed a decent heat recovery system from his log burning fire using two flues many moons ago, works very well. I'll annoy him tomorrow with this little conundrum

I also happen to have 30m of underfloor heating pipe knocking about in the shed