I think I would go in when you're both feeling calmer and say "DD, I heard you were upset. I'm sorry I was cross earlier. Do you want to tell me what's upsetting you? I'd like to help you, if I can."
If she tells you what the problem is then great.
If she won't talk, then I think you could help her anyway. Make the assumption that she was upset because she really DOES think it's impossible to keep her room clean for a month.
Think about this - you offered the trip as a motivation, to show her she can keep her room clean, right? You have the money for her to go. You WANT her to go. You haven't offered it as a fake carrot, to yank away at the last minute and go "Ha ha, knew you couldn't do it, now you'll think about cleaning your room!"
So start with - someone's natural level of cleanliness is not a moral failure. The state of her room is upsetting to you, but it might literally not bother her at all. The honest truth about why she drops things on the floor is probably that it is the easiest option and it doesn't seem to make a difference in her mind. Try not to get stressed about it nor to see it as a slight on you, or a sign of disrespect.
You could go in tomorrow morning, or talk to her tonight, and say look, DD, I know it's hard for you to keep your bedroom clean. And I know I get cross and perhaps that's a bit unfair. Tomorrow is Sunday, so we're going to spend the day sorting your room out and making it easier for you to keep it clean. I want you to achieve that goal, and I know that you can do it. I believe in you. I want you to have that experience of a month's worth of a clean room to see what it is like.
Open the window while you're cleaning to get some of the stale old air out. Put this podcast on (it will give you a great insight into what she might be struggling with, and it will give her some tips she can use). I apologise, I can't remember if there is any swearing in it. The podcast author does occasionally swear, mostly in ways like "what the fuck" rather than anything directed at anyone.
https://www.strugglecare.com/podcast-rss/04
I know that you want her to do it herself, and I know you've done some cleaning/organising in there recently. But I think that creating organising systems WITH her will probably help a lot. What helped my DS was taking a load of old stuff that he no longer used/wanted out of the room, so it's literally just his bed, chair, computer, desk, school stuff, books and clothes and a couple of figurines for decoration. Plus a bin and laundry bin.
That way when he tidies up he has a clear order of tasks to follow and they are all manageable. We also check in with him regularly (Sundays) to remind him to tidy up, and this helps a lot too.