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The electrical system in a campervan can be suprisingly complicated. Most vans will have an 12V auxillary or 'leisure' battery that runs the internal electrics (lights, pump etc). This is then charged either from the engine whilst driving, or from a 240v mains source when hooked up at a campsite. To control the electrics a distribution and charging unit is used which generally provides charging for the leisure battery from the mains and ignition system. This also provides mains power to 12V conversion, several individually fused 12V circuits as well as a mains feed to a fridge. The most reputable manufacturer is Zig Electronics and their 'Zig units' have become the most reliable and safe method of wiring all the circuits together. Some vans will also have mains outlets when connected to a suitable supply - for safety these need to be fused too.
There is a large range of power available from campsites and this can greatly affect the appliances that can run properly. Some sites have very low power - 3amp - supplys that won't run a fridge and charge the battery at the same time. Bear in mind that a fridge on 12V supply can drain a standard car battery in a matter of hours.
12V Electrical circuits will generally include a lighting ring, a supply to power, an electric pump as well as some 12V outlets suitable for phone chargers and the like.
With the correct distribution and charging system it is possible to go for quite some time without charging a leisure battery provided that there's enough driving to keep the battery full. However for long periods in one place it is necessary to hook up to the mains.