DC LV Battery Power Supply for Motorhome

If the system is just 'safe' voltage, e.g. 12 V, then there's simply no point considering any earthing for safety reasons. In that case the only point in 'earthing' is in the sense of the word commonly used for vehicles, meaning connecting to the metal chassis (not the actual earth) in order to use that as a conductor in the circuit to save on wires. If you're happy to use separate wires for the whole circuit (not the chassis), you'd probably be better to leave the whole 12 V system isolated from the vehicle and earth.


It's only if there's a source of dangerous voltage involved (e.g. external hookup or internal inverter) that there's any reason to consider connections to  the chassis (other than the functional reason above) or to the actual earth.  In the case that the dangerous voltage is from an internal source and no part of the wiring will leave the vehicle (fixed internal load), there's no point in connections from the vehicle to the actual earth. 


Regarding the original question: the only protection you need for the 12 V system is to avoid danger from overcurrent, i.e. overheating during short-circuit or overload. You should be fine to follow vehicle conventions.  It should also be fine to follow mains installation conventions for overcurrent protection: most modern devices (e.g. domestic-type MCBs) are considered to provide full protection to wiring that's rated no more than the device's nominal current.  It was mentioned earlier that breaking DC is more difficult. Some modern MCBs do have a rating for DC systems, that would be more than adequate for your purpose. I'd happily assume any 230 V MCB to work fine with a 12 V battery (interested in any demonstrated case of this not being true), but if you feel a need to show that specifications are rigidly being followed than just look for one with a DC rating.

If the system is just 'safe' voltage, e.g. 12 V, then there's simply no point considering any earthing for safety reasons. In that case the only point in 'earthing' is in the sense of the word commonly used for vehicles, meaning connecting to the metal chassis (not the actual earth) in order to use that as a conductor in the circuit to save on wires. If you're happy to use separate wires for the whole circuit (not the chassis), you'd probably be better to leave the whole 12 V system isolated from the vehicle and earth.


It's only if there's a source of dangerous voltage involved (e.g. external hookup or internal inverter) that there's any reason to consider connections to  the chassis (other than the functional reason above) or to the actual earth.  In the case that the dangerous voltage is from an internal source and no part of the wiring will leave the vehicle (fixed internal load), there's no point in connections from the vehicle to the actual earth. 


Regarding the original question: the only protection you need for the 12 V system is to avoid danger from overcurrent, i.e. overheating during short-circuit or overload. You should be fine to follow vehicle conventions.  It should also be fine to follow mains installation conventions for overcurrent protection: most modern devices (e.g. domestic-type MCBs) are considered to provide full protection to wiring that's rated no more than the device's nominal current.  It was mentioned earlier that breaking DC is more difficult. Some modern MCBs do have a rating for DC systems, that would be more than adequate for your purpose. I'd happily assume any 230 V MCB to work fine with a 12 V battery (interested in any demonstrated case of this not being true), but if you feel a need to show that specifications are rigidly being followed than just look for one with a DC rating.