look into installing a “Split Charge Relay”, a Leisure Battery, and then an Inverter.
also look for 12v LED strip lighting on eBay for your lighting direct from the battery.
You shouldnt really need much more than 500-600W inverter (I have a 400W which is fine for a couple of computers or charging a portable DVD player)
Budget around £100 for a 100Ah leisure battery, maybe £20 for the LED lighting strips, and I cant remember how much the split charge relay kit is, but I think eBay were about £40-50
MORE:
The split charge relay takes 12v power from your alternator while driving SO LONG as your car battery is fully charged. This 12v can then be connected directly to your leisure battery to drive it.
You can of course charge your lesiure battery while at home using a normal car battery charger.
The inverter converts 12vDC to 240vAC. Dont be tempted by too cheap and too low wattage a unit. Budget a good £50