Mormor and Morfar have a camper that we usually drive around in while we are there, since it's got space for all six of us at the same time. It's actually rather nice traveling this way since the ones who sit at the back then have a table, it feels a bit like a train compartment.
This is how we do it - kids play their games or watch movies on the Archos, Nathan plays with his iPhone and I read magazines!
Two years ago the kids and I spent a short week with Mormor and Morfar in southern Sweden, sightseeing and living in the camper. It worked out alright for those few days and ever since the kids have been nagging us to sleep in the "truck" again. I'm not so keen on it anymore since they are both bigger by now and take up that much more space - but Mormor offered to sleep with them one night, on the yard!
Although it turned out to be "the sleepover that never happened", 45 minutes later they were back inside again after Mormor had had enough of their bickering and fighting... Maybe next year!