If you have one or a few commercial vehicles that regularly keeps getting a flat battery due to the interior lights being left on, then maybe an additional battery is what your looking for. Leisure batteries work on the same principal no matter if you have a commercial van or motorhome.
They take there power from the main battery whilst the engine is running or via a mains hookup system. The idea is that once the engine is switched off, the power can be taken from the leisure battery safeguarding the vehicles main battery.
There are many items that could be ran from a leisure battery in a commercial vehicle. For example lights, fridge, coffee machine, power tool chargers, water pumps, mains 240v inverters and laptop charges to name a few.