Here is the standard wiring for a simple split charge relay system.
Lets start with the negatives of this system:
You really want all negatives to be common – shouldn’t be a problem with a relatively standard vehicle (if they aren’t then you need cables capable of handling the maximum alternator current to create a common negative. )
If the leisure battery is really flat when you start the vehicle it will drag down your main battery when the vehicle is running (so use battery monitors! and if it is flat don’t start the engine then stop it quickly, allow the charging system -alternator- to top the leisure battery back up first (you can also fit a battery isolator too!)