12V inverter to power laptop,
SLA battery + charger from
www.batteryworld.com.au
Female car cigarette lighter sockets, inline fuse, wiring and assorted crimp lugs also available.
Or buy the parts from Jaycar and build a DIY power distribution board.
Work out what the peak power draw of your mount, inverter and heaters amount to; the battery must be able to supply that as a peak current.
The second question is how many amp-hours are needed overall (ie on average) (amps x hours, understand ?). This is less than the peak current but I'd multiply by 8 to 12h to make sure it has plenty of capacity.