This is one case where it probably makes more sense to refer to "ground" rather than "earth". In a rolling vehicle there is no connection to earth unless you're plugged into shore power. The vehicle chassis is ground but it isn't earth ground.
Ground also implies earth to me. They are different words for the same thing. Ground, is more commonly used in the US and Canada and earth, in the UK and Australia. For example, in the UK modern cars are referred to has having a negative, rather than positive earth, as was the case for some vintage models.
If you really want to be unambiguous, you should refer to 0V, common, or chassis, rather than earth, or ground.
Ok so I did get wrong terminology,
I agree the RCD will not work without the earth and the way I was going to overcome this was to connect the neutral direct from the inverter to an earth wire giving the three live earth and neutral connections. This will then pass through an RCD and to any appliance so the appliance will have an earth. I do not see any issue with this at first until you get a live conductor short out to the vehicle bodywork then there is a potential lethal voltage on the bodywork which will not be detected until a person touching this body work then touched the earthed appliance.
To over come this connecting the earth/neutral connection to the chassis will make it all safe, but then someone pointed out some inverters are not true isolation between input and output so when the neutral switches to high it will be a short to the battery negative.
On more expensive inverters there will be an isolation like a transformer I expect. So cheap inverters are either to be kept with no earth to make them safe or do not use them
The problem is, it's possible the output from the inverter floats, at a DC voltage, with respect to earth/the 0V input terminal, which would result in a short circuit, if you connected the neutral to the chassis.