The long and short of it is I have a large, multi level dungeon planned as the major part of this section of the campaign, but I don't want my players to just try and brute force it, because I am certain they would get bored (it happened in the end dungeon of Lost Mines of Phandelver) but they're too stubborn to leave. So I was thinking of ways to encourage them to go back to town and then stock up to do it again.

My idea was that long resting in the dungeon does not stop exhaustion from building up. So they are not being forced to leave but it would do their advantage to do so. I was going to justify it as "the extreme latent nercromantic energy is sapping away at your life force, you need to go talk to your priest friend to have it cured." Or something like that. Is that too much? Do you have any better ideas?

TL;DR How do I convince the party to take the mega dungeon in chunks?

EDIT: Thanks for all of the comments, these are better than anything I could have ever asked for, I really appreciate it. I wanted to clarify some things that a lot of people have mentioned so I don't have to keep repeat myself.

1. Why not give them resource limits? (food, water, etc)

A. These people have been living off of goodberries and produce water for months now, and it feels disingenuous to stop them from using those spells.

1. Why do they need a break? Isn't going through it one go fun?

A. This party is...weird. They get into cycles where whatever they are doing is "the thing" and then will keep doing it even if they admit to me that they are not having fun. They want variety without knowing how to create it themselves.

1. Why not try to make it harder to long rest?

A. I've tried. In the ending dungeon of Lost Mines of Phandelver, they tried to long rest any time they were minorly inconvenienced, and even when I would always through hoards of monsters at them, they took that as "let's spend two IRL hours tearing the dungeon apart to make a fortress for them to sleep in, to the point where I pretty much only had the option of saying "it doesn't work because I said so" but I hate doing that, so instead I just let it happen. I need a good reason beside enemies to get them to move.

I appreciate the suggestions, and without knowing how stubborn they are, they were all perfect. I hope this helps frame my idea.

EDIT 2: For those asking, yes it is quite similar to dungeon of the Mad Mage, but when I floated the idea of the campaign to them they said it was to crunchy and had not enough story. So I am adding a lot of plot and character specific plot threads to make it relevant and story driven, and the dungeon is suppose to be 90% of the combat of this "Act" (I hate there isn't a better word for it) of the campaign, and the rest is a story about personal demons, past mistakes, and cult intrigue, as almost everyone in the party (by sheer accident, and I am not looking a gift horse in the mouth) wrote in the backstory that they had cult affiliations. Think of the dungeon as every combat encounter over the course of a written campaign smashed into one place.