Help needed: upgrading van

Hello CW readers.I have a 2003 Jayco Freedom Poptop and want to put a battery system in it.I have no idea how to go about it or where to go.I live just south of Brisbane so if anyone could advise me on where to get it done I would be grateful.If it's relevant there is an anderson plug at the plug that goes into the car and it has a centurion inverter in the van.Also could anyone tell me where you would buy new innerspring mattresses for the single beds.Cheers Geoff.

Geoff Austin, Qld