I know exactly what you mean, sounds incredibly familiar, and yes, very normal behaviour for those children who are in more need of stimulation than others. Unfortunately, kids at that age don't put their behaviour on hold during holidays, actually often are much worse.
I'll say something that I'm sure is controversial, but that I've heard from three different teachers and that is that kids who are demanding in attention as often the brightest ones because their being demanding is a show of their need to be mentally stimulated and that comes with some level of manipulation, ie. seeing how one reacts to their different behaviour.
Two things come to mind with the example of packing down. Firstly, 2 hours of waiting is a very long time for a 5yo, even if spoken to, so it is no surprise that she would have kicked off after some time. Also, it is easy to get involved in what we do and start interacting more with the other adults without realising and only the child notices and therefore acts up to get the attention again.
What I've found with my children was that if we were going to be in any situation where they would need to 'wait', usually travelling, I had to plan every 15 minutes of that wait and ensure they would be kept busy. That's the other thing, neither my kids have much imagination, they don't do imaginative thinking, and certainly don't do 'happy to just watch', so they needed to have something to do all the time.
The second thing is does your partner ever mention consequences for her behaviour? This worked a treat with my son. I would tell him from the start what the consequence of bad behaviour would be. It helps to say it before getting fed-up because you are more likely to come up with something reasonable (the things we shout out of anger and then realise we can't carry out!), and also because this way they are aware. Of course my son challenged me to test whether I would go ahead with them a few times. I made sure I sticked with it each time, even when it turned out to be a real pain (had to cancel at the last meeting up with a friend I hadn't seen for months when we'd arranged for a day out to the zoo, which I felt very bad about, but she understood the importance of sticking up to it) an that worked wonder. After that, I'd just had to mention the word consequence and he knew what he was risking to lose.
What makes it especially difficult for you is the fact that she is not your child, so it's hard enough to adjust to a life with a child, let alone a child who requires a lot of attention, when ultimately, you picked the relationship with your partner rather than the child, even if you accepted they came as a package. Do you and your partner ever get the chance to spend time just the two of you? If you did, I think you would find your partner's child less overbearing.
Whatever you do, don't associate her negative behaviour to her persona because otherwise, you are likely to slowly grow to dislike her. Instead, take that behaviour as a sign that she is an intelligent child with whom you will soon have a lot of fun interacting with. As long as your partner tackles the behaviour rather than ignore it, she will likely grow to be a lovely child and adult with whom you could have an amazing relationship. You will both be able to look back and laugh at her attitude (as my teenage son and I do together now).