There are plenty of split charge relays on Ebay, from £6.99. (
Like this one ) You can get automatic ones that need no trigger wire from the alternator (blue wire) = easier to fit though I prefer your more standard relay. 70 Amp size is about right. You can use a smaller standard relay tapped into the trigger wire to allow the fridge to get 12 volts ONLY when the engine is running.
Cannot comment re putting a leisure battery there, it would be neat if there is enough room, but I suspect you won't be able to find a large enough capacity battery that will fit in there. imho, if you are going to fit a leisure battery, you want a big one that will not run out half way through a rainy evening whilst watching the TV !
I just thought, doesn't a Westy Joker already have all this stuff built in, or is it gone or faulty ?
I also just noticed that was your first post, a "Hi all" post would have been nice but not to worry
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