First, you need to create your own happiness, and you're not doing that. You're letting other people do that for you, and obviously that's not working. But that doesn’t work for anyone! Sure crap happens and some people will always act like crap towards you while some will help and support you, but ultimately you, not anybody else, are responsible for making your life happy & what you want it to be.
Don't go on the vacation. The person doing the inviting, arranging, and the paying can ask anyone they want to come along. And it's not really fair to complain about your B&SIL inviting your MIL to help them when the two of you want the trip so you can check out housing in SC. You don't like that the inlaws are using your MIL for help and money but you're going to use them for a cheap trip and a chance to prepare for a potential move. It sounds like you'll be miserable but you also want time alone with FI- stay home, be happy, and enjoy a week alone with FI.
And move out. I can only think of a few good reasons why it's ok to live with your parents/inlaws as an adult (things like your house being destroyed by a tornado, fire, etc). I moved back in with my parents when I was laid off from a job and I took a much lower paying job. It sucked. I resented the situation and so did my parents. I really should have taken on a second job and kept my independence. Besides, the two of you need to experience living on your own- either each of you alone or together but alone. You need to know that you can deal with his family when they're not in your face. If you still can't, then I agree, maybe you shouldn't get married.
And I can think of half a dozen places where I grew up that are cheap and safe- under $500 a month (Cheviot, Ross, Western Hills, Delhi, etc). Sure they're not spectacular neighborhoods, but people also aren't dealing drugs on the corner, shooting each other, etc. The apartment might be basic, the people might be a little rough, but they're all probably going to be good, working class neighbors or seniors. The downtown area is either going to have expensive, luxury places or places for people on federal assistance, there is no in between.
As I see it, you can either stick out your current situation and try to be happy with it, or you can create a better situation and be happy with that.