Usually your gut is right. 

What do they do to make you think that way?

Follow your instincts!

Always!

She has a  mother and you are not it.   I would never sit home and let him go on a bar crawl.  Either I go or he stays home with you. 

Don't let your fiance gaslight or create cognitive dissonance. He's trying to reshape reality into his Happy Family fantasy.

In step relations, and especially with females, there can be a thin veneer of sugary civility on top of a lot of negativity. Because men often don't understand nuance or are willfully blind, they don't recognize the subtle snubs, jabs, and instances of discourtesy perpetrated by their daughters. 

You know what you know and are experiencing, and you have every right to distance yourself from people who aren't genuine or don't wish you well. Don't allow your SO to create self doubt or put spin on his daughters' behavior. He has an agenda (squishing all the females he loves together into a family), but you are all adults who get to decide the level of interaction you have with each other. You can't rely on his judgement or perception, so don't let him badger you into things.

Do I understand that he's going pub crawling with his daughters, and on a trip to LV with them? Does he seem a bit enmeshed with his adult daughters? I can't imagine my H partying with his like this. Observe his level of involvement with a critical eye. Trauma like death of the other parent is a common factor in enmeshed relationships. Don't go forward with a wedding until you feel confident that your partner is clear-eyed and has healthy boundaries with his daughters.

Oh sd’s want to meet you and our toddlers, they miss you all. So I sarcastically replied “is that why they haven’t visited or contacted us in 3 months? 6 months? Currently we are at 9 months and approaching 10 fast... yes they do miss us huh?? That shut hubby up real quick. Truth hurts and i told hubby don’t ever bullshit to me like this ever again... 

I also suspected my SD didn't like me, but my husband assured me his daughter liked me and was happy for our upcoming marriage.

A decade later, I disengaged and haven't seen SD for more than two years. Our marriage has suffered because of SD -- she's the only thing we ever fight about more than once.

I often try to figure out why I didn't trust the evidence of SD's actions and words instead of what my then-fiance was telling me. This was when I discovered my husband doesn't know his daughter at all or won't be honest about what he does know. I should have watched out for myself, but I trusted he'd never let us just marry without working out big issues like this first. I should have trusted MYSELF.

After busting my ass for SD's wedding, I thought we'd mended our fences. About six months later, it all started again: holiday cards addressed only to my husband, holidays gifts for her dad shipped here without even a card to me...it was as if none of the wedding even happened. When I brought it up to my husband, he said "She just forgot to put your name on the card." I KNOW he KNOWS it's more than that, but he just can't bring himself to see the truth without rewriting it.

Why he doesn't see the truth doesn't matter. You have to see the truth and figure out (on your own) how you'll deal with not one, but many stepdaughters! None of it is in your head -- don't you remember junior high school??????

I didn't party with my parents. I think that's weird. My guess is your boyfriend is his kids' friend, not his kids' parent, so drinking with him is fun for his kids. But when they drink with you, it's like having an old lady tag along uninvited because they don't really want you there. If you remember from junior high, that is always the kid who is picked on -- the one who tagged along uninvited. They're drinking with friends, they want to have fun, and you are a giant target to play with.

You can disengage too, but you'll soon hate that he's always out with them, and once they know you won't be coming along, the invitations will really flow! You're in a bind to be sure, but the first step is recognizing that HE is not assessing the situation for your future happiness. He's assessing the situation on HIS future happiness, and he's not even doing a good job of that -- someday, when his kids are really drunk or out of hand, he will be shocked by their bad behavior toward you, but even that won't make him do the right thing by you.

So please, don't let anyone tell you it's in your head. Don't let his reassurances confuse you or make you push the problem aside. You probably can't fix his kids' nasty attitudes (I never could), but before you marry, you can set expectations, negotiate how things will be handled, etc., so you are protected. Don't expect him to protect you -- he won't.

I definitely would not marry him without knowing a proposed exit date for his kids to move out.