S.C.
It is not a bad behavior. Think of kids that leave a trail of toys across the den, bedroom, driveway and hallway! Maybe have a rule of what can be collected, kept, given away, stored. We ALL have to deal with this!
What you might try is to allow her certain places to store her items like get a inexpensive plastic bin or three of them and allow her to fill up to lifes content each of these bins (with lids) And one or two portable bags that one can be for emergency travel and the other for future "travel". I will give you an example. For business travelers for them to repack from scratch a suitcase every time they are told or need to travel somewhere is silly not to keep it pre-packed and ready to go. So the same with your 5 yr old. She needs to have her ready to go pack but with the things that need to be in there. For example old food will spoil the bag while a book could be there along time. But also tell her we need to review the contents of the bag to make sure they are still useful evey week/month or quarter. Same with those 1 - 3 boxes with lids. Best to have ones that are transparent so can quickly assess what is in them and how full they are. There are stories about a man who lived alone and is supposedly true. Saw on TV who was such a pack rat he could not get down the stairs to leave the house safely. He was dating and never could invite his date to his home. It was on TV and he had to get help with his hoarding problem This we dont want to happen to anyone. So this is the time to monitor, control, and teach the child good storage and disposal and value of the space use. For example:
If a bag was full of air, vs full of old chicken nuggets, vs full or shredded paper vs full of cherished bears. What the child Values/Wants will of course change over time and probably differ from what you value. We have a 10 year old girl now. Never moved or what not. But now she has about 5 containers of her stuff. It is better than all over the house!
It is called compromise. PS in Califonia they are trying to ban selling fast food with the incentive of a toy. That might help if nationwide someday.
Kids to trade these food toys. Some parents just tell their kids if you dont do such and such you have X time / days to clean it up or you will decide what is needed or not. Called tough love but that is not what you really want to do! Vs teach, reward, be positive with the child how it helps them to find their things, how things are safer when not crammed into a bag. I will give you an example for me. Am getting a Touch Screen netbook and read in the instructions they are pretty sensitive and to not close the keyboard and lid with the lcd screen with something like the stylus or whatever stuck between the keyboard and the LED LCD screen as it could stress it. So traveling with one of those which it is made to do, and say placed into a sack or bag it must be given its own space and not crammed in with the power adaptor etc pressing it till it cracks. So the same thing goes for your 5 yr old! If she crams five happy meal boxes into a bag they may get crushed and no longer like they were. Kids need guidance and key rules to follow.
As for your Edit: Yes kids freak out and protest but again they need rules and guidelines and reinforcement and warnings before your "clean" them out. Think if She went into your bedroom and started to take your "old" shoes and stick them in back of a closet how would you react? What you value is different than her. But the key is teaching her to see things the way you do and over time this should improve. Also explain that if bags collect under her bed that she cant get to them very easily. But another solution we did: We got roll out boxes like sold at Target, Walmart etc for blankets and shoes. Let her fill them up but first explain that space may be needed for her such and such first. Dont stifle this but mold the behavior. Tell her you value her dong such things but we must learn how, when, why, etc we do such things and how it helps or hurts us. She may still freak out but at least you have a Mutual Reason to recall to help her calm down, maybe.