With the extra load put on your system by running additional 12 volt equipment, a good quality leisure battery is a must. Fridges, additional lights, even radios all add to that power drain. Opting for a battery which has as many 'Amp hours' (AH) as possible within your budget is a sensible choice. Our suggestion is not to drop below 95AH which will give you longer life between charges. Don't forget to buy any clamps you may require (quick release are always useful). Popular brands include Lucas, Exel, Hi-power, Varta, Elecsol, Platinum, Halfords, Tayna, Numax and Trojan.
Look for 'deep cycle' on the battery as this is designed to provide a consistent voltage as the battery discharges.