I have done it on my van, I did put a post on here some time back with photos, but I can't find it.
My solar charge controller switches a relay on and off which controls the 12v that is feeding the element like you have shown, the element is in the side of my truma water heater tank. I have incorporated a thermostat that also switches the relay should the water get too hot. Here is my post of another site, I have upgraded the cables since the photos where taken.
My final Mod I think, 12v solar hot water - tribby.co.uk