J.C.
I'm sorry to say this, but your acting like a selfish brat. Stop it!
Those are your husbands parents and regardless of how you feel about them you have an obligation to no only visit them and allow their visits, but to act happy about it. You cannot hold a grudge against them for something your husband did (the mortgage), that's really not any of your business. And I would HOPE that your husband would pick up the tab for his parents no matter how much money they have, that's honor! But especially since they don't have much, you should GLADLY pay for their meals and entertainment during your visit.
Now that you are TTC you should think about how you would feel if your son married a woman who hated you, didn't want your son to visit you, didn't want to visit your son, and wouldn't allow you to see your grandchildren. Yikes!
Treat them like you want to be treated by your children and their spouses.
You should really go apologize to your husband for acting like such a pain in the butt and tell him you are definitely going and you will like a mature adult while you are there. If you want I see no harm in asking your husband if you both can stay in a hotel during the visit so you are more comfortable.
Yes, they will probably want to come see their first grandchild when he/she is born. That's a good thing! If you don't want them to stay with you then make hotel arrangements and YOU pay for it. Make sure its a nice hotel and not a motel 6. Then you can also have some control over the length of their stay. You can say something like, "Hey mom and dad, we would love for you to come visit when the little one is born. We've made arrangements for you at a nice hotel for a week visit! Don't worry its all paid for, look forward to seeing you."
Then you are including them, acting like a mature adult and not a spoiled brat. They will appreciate you and love the "new you" and perhaps you will find they aren't so horrible after all, just loving parents.