I agree regarding any MH issues he has maybe explaining some of his behavior, but, IMO, it was not a primary factor in him committing the premeditated murder of his three daughters in cold blood and then disappearing into the wilderness.
Also AFAIK, because he only had visitation with them for 3 hours during the dinnertime hours (5 - 8 pm), he was only supposed to be taking them "out to dinner" as before.
So taking them to the campsite he apparently had stayed at the day before (and only that day before was mentioned, IIRC, maybe based on cell data from LE?), would be an anomaly and not be in keeping with agreed upon plans.
Maybe he told the girls they were going to do something different and cook dinner at his campsite because maybe he admitted he was too broke to take them out to a restaurant, and wouldn't it be fun to roast hotdogs and marshmallows over the fire sort of thing, as a ruse that wouldn't be too alarming (I hope)?
I don't think he had truly set up camp there, though, and had maybe set it up the day before as a staging area to take them the next day and kill them.
I don't think it would have made sense to the girls for him to pretend they were all going to camp out, when they would have known that would be out of the scope of the visitation arrangement, which was dinners out with dad, AFAIK.