The new 12v fridges work using a compressor the same as the one in your house so when it gets cold enough it stops running,the leisure battery is separate from the lorry battery. The worst case scenario i imagine is if it was grey-rainy but warm weather - your fridge would have to work hard to stay cool and no sun to keep the battery topped up.A 12v connection to the lorry battery is handy to charge the leisure battery while the engine is running.