So in my campaign, there is a dungeon with a bunch of haunts, but they are all tied to one main haunt. Now if they skip to the end, defeat the final haunt and dispel it, should they get XP for the haunts they didnt defeat? Or do they just miss out on it all? Any help is appreciated.
IMO, They miss out on it.
XP is defined: "As a character overcomes challenges, defeats monsters, and completes quests, he gains experience points."
They didn't defeat the monsters, they bypassed them. The only one they defeated was the boss, so they only get xp for the boss.
Kinda depends on how they skip to the end. One way I could see a party skipping to the end is if they decide to go in the back door of the house instead of the front door, they get a back stairway directly to the BBEH (Big Bad Evil Haunt). In that case, I think they don't get the XP. But if there's only one way into the area, or if whichever way they choose doesn't provide an advantage in order to get in, and bypassing the haunts requires they discover the secret door and figure out how to rig the dumbwaiter to let them access the third floor then pick the lock to the attic and guess the riddle that previous haunts would have helped them guess if they had met them and then beat the BBEH, that's a lot more involved and they would deserve more XP.
In the end, though, it's a question of where you want them to be in terms of level at the end of the adventure.