I'd be seriously worried about that. You get all kind of weird expansion things going on.
Interesting what guy above said about the element in tanks usually being in the top - that's right, I think. Take a look at an immersion tank and you'll see the same.
Anyway, immersion tanks are made from copper/metal, surely, not plastic?
Why not, therefore, have two immersion (copper) tanks in the van for wfp water?