What did he do before your relationship? Who did the cooking and cleaning? If it's his home, he should be contributing to those tasks. And there's no way his kids should be your #1 priority. They should be his, and he probably just wants you to be all in so he can pawn the work of the kids fully off on you. Also, it's often true that men still tend to favor their kids from their first marriage if they have kids with someone else, so I'd strongly encourage you to find someone who doesn't have their own yet, and even if they did, someone who doesn't think you should do all the work!

He has high expectations that you will take over parenting for him? If he can't handle his kids, ALONE, he should stop overnight visitation and have them for a few hours instead. 

It's really easy to say but have you told him you are not their parent? They already have 2 parents a mum and a dad you are there to back up your partner and possibly be the skids friend (do fun stuff) you are not there to clean up or cook for his children and he needs to be told. Being engaged with his children is not doing it all so leave that to him and you only do fun stuff if you want to. The skids aren't there to see you they're there to see their dad. Your partner needs a wake up call (as do most including mine) they are quite young hopefully they will grow out of most of the annoying stuff but you are not hired help to do all the jobs he doesn't want to do. 

What did his last slave die of?

These children are not your responsibility. He has no right to have "expectations" of you. As long as you are cordial and polite, he can't ask you for more. These are his kids and HE is the one who should be raising them - cleaning up after them, teaching them good manners, feeding them, etc. He has no business expecting you to take up the slack. Disappear for the next couple of weekends when they are on visitation. Enjoy your time. If he protests, ask him why he thinks it's your job to parent his kids. 

So often on Steptalk, we ask: has he done "the work"?  That means, has the dad ironed out visitation, do they have the right equipment (beds, refrigerstor), does he have behavior standards established and does he enforce them, are the children comfortable staying there, does he have the logistics taken care of (cleaning, groceries), does he have a custody order he follows.  All these things are HIS responsibility.  Im guessing he is depending on you to handle all this.  Its not fair to you. Its laziness.  Often, people want the "happy family", thats shorthand for "you do all the work".

Why do you stay?   It doesn't sound like this is the life you want.  What do you have to lose by telling your partner that this arrangement isn't working for you, and if he wants to continue the relationship HE needs to take over all parenting responsibilities and change his expectations on your role with his kids?  Either he steps up and you end up with an arrangement that is acceptable to you, or you find out right now that nothing is going to change and you can either resign yourself to that or make your exit plan.

If my now DH treated me that way when we were dating after a year I woulda been goooone. We've been together 7 years now and are married, have a home together and his 3 children half the time, and over time I started to do more and more for the children, because I wanted to, not because he was expecting me to or making me do it. If there are days I don't want to cook him and his kids dinner I don't, and he does, without complaint. These are his children at the end of the day and 100% his responsibility. He should be thankful for any help you offer, he should not be expecting it or making you feel like a slave. 

I think you need to take a step back and let him step up and see how he reacts.