As I see it, there are 2 ways to do this, the bodge way using standard 13 Amp plugs, which could lead to injury, fatality etc so this would be illegal.
Then the correct way, as seen at camp sites and my own personal experience on shore power on boats.
This must be correct as Electricity + Water =
In my opinion from observation:
You need a Blue 16 Amp Socket ( OUTLET) on a suitable wall or enclosure that is fed from the house or garage via an earth leakage trip (RCD) that disconnects both the Live and the Neutral.
This should also have a circuit breaker of Max 16 Amps.
You then need a Blue 16 Amp plug (Inlet) mounted on the car. Best to have the surface mount ones that have a cover over them to protect the pins.
I would then suggest a garage type earth leakage trip (RCD) that disconnects both the Live and the Neutral. Mounted inside the vehicle, with suitable low current breakers, say a 6 Amp for fridge, lights etc, and a 16 Amp for power sockets.You are then free to distribute mains power using standard sockets. The cable would be best installed in plastic conduit.
You might want to put in a blue 16 Amp socket outlet as well, so if you only have one supply at the camp site, you can then feed the caravan from the car socket.
WARNING...
I know this is obvious BUT make sure with the pins and sockets on the Plugs / Sockets are the correct type. You don't want a situation that any accessible pins are ever live, no matter how you could connect them.
Discussion point the part in
blue above is debatable, but consider if ever you connect a generator to the car, then there needs to be some protection.
In fact the earth on the generator would require an earth rod etc.
For peace of mind, it is worth getting one of those plug in devices that shows 3 lights if the wiring is correct, as it has been know for Live and Neutral to have been crossed over....
All wiring would have to be suitable for the installation and fuse or breaker protection. On boats etc, Standard twin and earth is not used, ie solid conductors, as the vibration can fracture the single conductors.
The above is only my opinion, and from what I have seen.
You will need to discuss this type of installation with a qualified Electrician, who now uses the 17th edition of the wiring regs.
I hope this helps as a starter.
DO NOT USE standard 13 Amp extensions outside and leave them un attended..