This is actually something that was covered in the most recent edition
(17th) of the wiring regulations, in relation to power hook-ups in
caravans and motorhomes.

Basically the regs say that caravans and motorhomes must have metallic
bodies/chassis earth bonded, along with RCD protection on the hook-up.

Presumably the reasoning behind bonding is that should there be a
fault that caused the body to become live, it could present the most
dangerous scenario of someone barefoot outside the vehicle touching
live bodywork.

That's all well and good in places like UK campsites with properly
tested hook-ups that have both a good earth and RCD protection.

It's not so good on places where there's a dodgy earth, or the
possibility of live/earth being transposed.

So - practically - do what you feel is safest in the locations you
will use your charger hook-up. Not bonding and carefully protecting
the cable sounds reasonable, if you're the only user and use it in
locations you trust.

An approach popular with many van users is to use a "tent power hook-
up". This is basically a mains plug on one end (often a commando) and
a mini-consumer unit on the other.

e.g. http://www.camping-caravan-outdoors....f.asp?item=876

With some vehicles (well VW T4 vans for definite) you can pop a
reflector out of the rear bumper and feed the plug and lead from the
consumer unit outside the vehicle, or even mount a connector there.

On untrusted locations, one of those mains test plugs to check
polarity of the wiring and the presence of some sort of earth, is a
cheap and quick way of getting an indication that all may not be well.