Hi Tyniclogs!
I have a 4yo DD and my theory about sudden displays of aggression, tantrum-throwing etc, is it's often to do with insecurity. With DD it often starts with cheekiness, but with an underlying edge of tension or aggressiveness behind it. Also any tiredness just makes it worse.
She is usually a normal, happy girl, but last year, our family holiday was almost ruined - it started off with her having night terrors each night, to the point of screaming blue murder. She also didn't sleep as long as she did at home, so each day, she was getting more and more overtired. By the last part of the week she was in right state, tantruming 5-6 times a day, over anything and nothing. We weren't in great shape either! Your post about your Mum made me chuckle as my MIL also suggested that we take DD to 'see a doctor', because we 'need support as a family'. After ignoring my repeated warnings that she needed more sleep than she was getting, and not being very helpful in helping me get her to nap/go to bed at her normal times!
It was the first time we'd ever seen DD behave in such a manner (at one point she even had a mega-tantrum in the car while DP was driving, and was screaming and kicking his seat so hard he had to stop the car.) Shocking at the time, but if it's any comfort - as SOON as we got home and back into routine, she was fine again within 24hrs. She was just incredibly homesick and for some reason, even though she liked being on holiday, the change affected her very badly. This year, she was much better - which also shows how much better they cope with situations, from one year to the next.
Recently she went through another phase of being a bit rude, and 'cheeky' with the same 'tension' underneath - and I know it was to do with nervousness about leaving nursery/starting school. This is because it happened around the same time as nursery did a 'graduation ceremony' and they'd all put on a show. The kids had all worked on it for weeks and weeks and I think it sort of freaked them out. Two days after the 'graduation' she wet her bed for the first time since she was potty trained. I talked to a couple of other mums, who also mentioned their children had started having the odd 'accident' too!
Anyway, I bought a couple of books to read with her about starting school, and talked to her a bit more about it, acknowledging that in some ways it would be sad, but she'll make new friends, telling her it's OK to feel shy/nervous, and that I was too - and since then she's gone back to normal. Today she made friends with a random, similar-aged little girl in a playcentre, and I said to her that's really good, that she's getting more confident at making new friends. She said "Yes. Because I'm starting school!" :)
But yeah - my theory is to look for possible causes of anxiety, and try and work on that. It's unfortunate that there isn't much you can do about a 7hr journey...maybe try and make the routine at the other end a bit more predictable??