The raps kit does not have the isolator we are talking about. The one we mean gives priority charge to your start battery, then links your two batterys together for charging. When you stop they disconnect so the only way your going to get a flat start battery is if you do something silly like leave the headlights on. That is a basic dual battery set-up that most people run when they have accessories like a fridge.
If it was me, I'd get the pre-wired box, glass mat battery, redarc solenoid off ebay and get some nice big wire off a car audio shop. Mount the solenoid under the bonnet and run the wires back through to where you battery will be and depending on the box you end up with, use an anderson plug to connect it for charging. Quick, easy simple and will last.
This is the redarc version -
http://www.redarc.com.au/smart-start.htm but there are many that will do the job.
Hope that helps.