Your vehicle earth must also be included in the protective conductor connection. If you use devices on the vehicle, they should of course also have and use a protective conductor connection … e.g. kitchen appliances.
You can use more than one appliance as long as they are all properly connected to the protective earth conductor.
As long as you are on the vehicle, this is not a problem. The residual current protection will function properly if the N conductor and the protective conductor in the Phoenix are connected via the bridge … see appendix in the instructions … are connected.
However, this all changes when you are outside the vehicle … An earthing rod MUST be used here BUT I would only ever use one device to avoid a so-called 2nd fault.
From my own experience, I know that these earthing rods are hardly ever used. Therefore, there is no danger in the event of a fault in a device … But with 2 devices and a distribution board in the outdoor area, it’s a different story. The so-called 2nd fault safety