I think for a 5 year old, you have to expect a certain amount of "bad" behavior at WDW, perhaps moreso than at home. However, that does not EXCUSE the behavior, but just means you have to have ways to deal with it when it shows up.
I would begin by setting a very basic standard for behavior on the vacation, with consequences for non-compliance. Depending on your family's values and expectations for the vacation, you can come up with what works for you. The consequence could be time out on a bench, going back to the hotel, skipping a favorite ride, etc.
When you feel that your daughter is getting close to a meltdown, act IMMEDIATELY. Remove her from the situation (whatever it is...a line, a show, a meal, whatever) and take her to a quiet location and ask her "what can I/we do to make your day better?" Perhaps the answer is that she is tired, hot, hungry, thirsty, wants to go back to the hotel, doesn't want to go on THAT ride, or see THAT show, etc. Try to LISTEN to your daughter, and be sure to take her needs/feelings into consideration instead of always directing the "show". A lot of times, us parents *think* we're doing all this stuff at WDW "for the kids", but in reality, we're running from here to there, to hit all the "do not miss" stuff that we *think* we should be doing "for the kids", but maybe the kids want to do something TOTALLY different, like just play in the fountains, or look at the flowers, or ride the train over and over again, etc. When little kids don't think they're being seen or heard, they tantrum to get attention. Consider that.
I would recommend asking your daughter every so often "what would YOU like to do now?" See what she says, and if it's a viable option, do it! You'll be surprised how much more easy going kids will be if they feel like it's THEIR vacation too, and YOU are listening to their input.
I have a 4.5 year old son, and we go to
Disneyland a LOT (we live close, and are AP holders). Sometimes, I'll ask him if he wants to go, and he'll say "no".

But, then if I tell him we are going, but HE gets to be the "boss" of the day, and decide what rides we will go on, and where we will eat lunch, etc. then his mood totally changes, and he gets all excited, and the whole time we are there, he behaves SO well, because he feels like he has a "job" to do (telling me where we have to go next). At 5, kids are sort of past the "tantrum" stage for the most part, and any bad behavior is an attention seeking instrument.