The thing to have in your mind is path - in a lightning strike the charge will take the most direct low resistance path.
Fun fact at the level of a camper the lighting strike will actually be going UP to meet the downcoming strike.
It's not really the Faraday Cage here, though the reason that they work is the same principle, it's that all the bodywork is electrically coupled so there is no reason for the charge to do anything other than flow through the metal, and the metal is a near perfect conductor so there will not be any voltage gradient. So at no point will YOU become a more conductive path.
At the energy involved rubber tyres won't make much of a difference, if the low current path is through the van body then the 6 inches of air is nothing to something opening a plasma channel kilometres long. Frankly neither will an EHU grounding.
I strongly doubt the pop top will make any difference to anybody in the van. Very possibly if you were in the poptop above the van body, but again unlikely.
BTW the reason lightning "conductors" work is not that they conduct a bolt to ground, they'd be instantly vaporised, but that with the sharp points at the top they allow the ground charge to conduct efficiently into the air so there is much less chance of an upward ground strike forming for the downstroke to find.
Your biggest risk in lightning is having your feet apart. Should a bolt hit the ground near you the resistance of the ground means your feet may then be many 100s of volts different and the current may take a path through you. It's why many sheep or cattle are killed as their feet are much further apart.
Best advice is stay indoors, if not keep feet together and try and take up the smallest space touching the ground.
What might be more at risk is your electronics from EMP. At sea we used to put phones and handheld radios in the yacht's oven as it was close to a Faraday Cage.
