the best ones i found were
http://www.auto-mate.org.uk/monocrystalline-solar-panels/solar-charge-controllers-regulators/voltage-sensitive-charge-relay bit expensive but the best to fit diy.......
the relay has 3 wires from it
red to van main battery
red to leisure battery
black to main van battery
then a black from leisure battery to any earth thats it no perm lives no messing about
Ideal replacement for a standard split charge relay but with an added bonus. When either battery bank reaches 13.3v it will join to the other bank allowing charge to be passed from engine battery to leisure battery or vice versa.
When wild camping the relay will allow both battery banks to stay joined allowing a larger battery capacity to be used. Once the battery voltage drops to 12.8v the relay will the separate both banks of batteries and allow them to react independently of each other. Stopping the possibility of flattening both banks of batteries.
These relays are fully automatic, fit them and forget them. No more switching your charger from one battery to another or a solar panel that only charges the leisure batteries.
*Not suitable for gel battery Systems*