I, like others, am not an expert on the subject however, I gather that children with austism prefer regular routines, possibly to the extent of obsessiveness. Any change, regardless of how small, causes a panic attack.
You mention mealtimes and I wonder if it would be possible to have some meals in the van, even if you are not away. This may allow the kids to get familiar more with the environment and not possibly make such a 'fuss' when on actual site. Also possibly try sticking to the same site for a while, again to get some familiarity.
As for the actual setting up you may need to follow advice given by 'David' and get one of you to do the setting up while the other takes the kids off to 'explore'. I am sure the more you and hubby get 'stressed' the more reaction from the kids.
Having 1 child with autism is a difficult situation, having 3 must be very stressful and I fully accept that I doubt if I could cope.
If it all 'fails' then you may just have to accept defeat and try something else, but I regret your pocket will feel the 'pain'. Possibly an alternative is to rent a static van for a break, at least all the setting up has already been done for you.