If you used an independent tent EHU (with local RCD) with it and all loads outside of the van body then as
@Salty Spuds notes you'd be in the same position as a tent and just as safe.
The risk comes when there is a chance of a wiring fault meaning that the body of the van goes live, for instance a EHU cable put through the tailgate/bonnet and catches on an exposed bolt or panel edge. As the van body is well insulated from the ground by the tyres there is no path to ground so no RCD will trip, but anyone touching the van with feet on the ground is a potential fault current path.
Normally this is addressed by grounding the van body to the protective earth of the incoming EHU, giving a path to ground, so any fault current will trip the RCD in the van consumer unit. A tent based EHU put in temporarily will not have this grounding so it's RCD protection will not work.
As
@Salty Spuds notes there is often an upstream RCD as well, however generally they will be slower acting to give discrimination - it's expected that the RCD at your end of the EHU will trip first if you have a fault, not the one in the distribution point that may turn off others. The role of the distribution point RCD is to protect people from faults in the cable between the point and your EHU system.
As in all these things you need a number of things to go wrong and line up for a tragedy to happen and in the UK regulations keep a number of layers present. The same is true of aircraft but they still have accidents when a number of unlikely risks line up and often it's found that some of the protection has been disabled or ignored as "unlikely"
Would I use a tent EHU in a van without grounding it? Maybe if it was once or twice a year (minimise the risk by not doing it often) and being very careful to check the wire for damage. I don't think I'd be happy doing it regularly, and I'd expect any commercial site owner who saw me doing it to ask me not to.
If you want to do this "properly" then you will need to replace your basic 16A to Powercon arrangement to the Clayton with a normal complete grounded EHU install with RCD and 2 breakers - one to feed the Clayton and one to feed an additional EHU only socket in the van.