Yes, I’ve done this on all the campers I’ve converted for years and it’s the only way to guarantee you will start in the morning… All those ‘charge relays’ they sell on ebog are hilarious, as they lack the ability to switch enough amps and simply burn out..!
I usually just use one simple battery isolation switch to cut off the leisure battery positive feed, as these switches come cheap (the ones with a removable key). I have used a marine double battery rotary switch to handle the batteries in the past too – they’re a bit more expensive and complicated to wire, but look really cool..!
Also, presuming you use wire of the correct thickness, you can use the leisure battery to start up if the starter battery fails. This happened to me on a lonely Spanish mountain just a few months ago – the starter battery had just got too old and died.