I am going to have a look into the dual battery and maybe use a yellow top deep cycle as a leisure for my audio, any idea if this would leave the normal battery fully charged for the starter even in the leisure battery died?
No it won't. The batteries are linked together and go flat together if the alternator output isn't sufficient. A split charge system assumes that you have an alternator big enough to charge both batteries simultaneously and power the load, it comes into it's own by disconnecting the batteries when the engine is stopped so that even though the aux battery goes flat, the cranking battery stays charged.
When you get into split charge systems and the sorts of current loads you are talking about, you need to pay particular attention to wire sizing and system design otherwise one battery will always charge and discharge much more "efficiently" than the other, which can result in you walking home.
If you are really serious about running a system like that, you're going to need a monster alternator and forget the split charge - I can't see where you'll get anymore advantage than if you did cutting out the battery box and fitting the biggest battery you can together with a big alternator.
Charging twin batteries in parallel (and in series for that matter) is a real dark art, and you'll never achieve much more than about 75-80% capacity, and that's with good system design.
