Thats the idea, There are several on the market ( you don't need dual sensing, in fact you don't want dual sensing) note that the battery has to remain above the 12.7 to 12.35 ( depending on make) to keep the relay closed, so if you have a load on the battery, the relay will not stay closed , but then thats what you want.
You only need a simple two terminal relay, positive from the battery to the relay , positive from the relay to the immersion, when the battery voltage is above the switch point the relay will close, connecting in the immersion , when it falls it will disconnect the immersion, This will work with all charge sources, obviously wire an on.off and a bypass switch to manually control the immersion,
Theres no need to switch the negative,
You may find if there is load on the battery that the relay constantly switches over and back, so its not a fully automatic system. more then likely the solar/wind will not generate enough charge to solely power the immersion, so the battery will continue, until the voltages falls and the relay trips out.
Blue sea also make some, with a lower dropout voltage too , it would seem.
voltage sensed relays, there are many programmable typos available through industrial distributors.
heres another http://www.the12voltshop.co.uk/Shop/...lay-60amp.html
a lot cheaper, though 12.8 is quite a high dropout voltage
True charge diversion/dumping really needs a custom solar/wind regulator
, love to, but currently sailing in france!