The earthing system isn't my concern. What worries me is that if the vehicle body is carrying the filter current and it becomes disconnected from earth at any point, then the vehicle body will be at supply potential, which could be 400V AC. Admittedly the filter will limit the current, but it will certainly be enough to cause a startle reaction, and it is vital that the correct kind of RCD is fitted in the installation, since the current will have a significant HF content as well as a DC bias.