I normally rate a split charge so it can handle the maximum output from the alternator to charge the second battery. Normally use either 100 amp/120amp relays and 266/0.30 cable(rated to 135amp). I've just finished an installation on a 110 with extra lights,second battery/splitcharge and winch if you want to have a look before the owner collects it(we're in Larbert). I run 2 batteries in parallel on my 127, which powers the winch, and use a 100 amp splitcharge to charge the 2 deep cycle batteries in the camper body. Work out what your fitting and its current draw and if the current alternator and wiring is up to the job of powering it all before you start.