Zs is a waste of time as you cant legislate for the power source. BRB does mention a max length for the 16A supply cable (usually damaged), take note of the warning notices that should be in place and also check out the earth bonding to the chassis of the vehicle. Insulation can also be problematic due to condensation and moisture ingress.
Stick to your usual standards, and have fun testing the RCD!!! (which one will trip)
With regard to the supply / hook up cable there are alot of visually in good condition cables that fail the IR test, a caravan engineer I know who has started doing electric checks as required by the NCC on caravans and motorhomes has seen quite a few failures
With regard to testing the RCD the only reliable way to do it is using an isolating transformer or most caravan workshops have a non RCD protected socket specifically for RCD testing purposes