Andy, this is called 'neutralising'. Most electrical companies connect the nueutral to earth at the consumer's meter somewhere and also at the transformer in all electrical distribution systems. Then the earth points are all connected to a system of buried earth mats or tapes to literally 'earth' the potential and provide a good quality low resistance earth connection. So in reality in an installation, neutral and earth are at 0V and thus the same potential. They are kept separate for safety reasons and occassionally neutrals can 'have' or 'acquire' a small voltage, particularly in 3 phase installations.
In your case the N and E should be connected together at the inverter, in theory, and then this connection point should be run to ground. If you want to be safe, get an earth rod in a wholesaler and a piece of earth wire, say 6mm2, and each time you park up to use the inverter, hammer your spike into the ground with the earth wire clamped to it. (this might be appropriate if you are parked up for a day or two but would be a bit unrealistic if you are only stopping for lunch). There's no real way of doing any better than this and the better the earth connection the more reliable the tripping will be on the RCD. You could connect to a wire fence or something if that was convenient using a clamp of sorts. Either way, it is a method of getting an earth connection of some description. I'd like to know how the original users do it in the vans they use. If you are using a small portable TV or similar, that in all liklihood is double insulated (no earth in the plug and you will see the 2 small squares symbol on the appliance name tag somewhere), then the earth bit is a bit superfluous in reality. A bit of common sense has to prevail but you cannot be too careful.
Didn't realise they recommended connecting the chassis to earth. Yes, you are right, just use the battery neg and it will connect all at equal potential.
