I'm not going to tell my in-laws that we are selling our condo and living in our RV full time.
Why do you think that you need to keep it a secret from them?
I don't want them to know and give me their opinions.
I can understand that. It's better to be honest with your family, though. It is a big change in your life.
What if I just moved in with them instead?
I am not sure that they would like that though. Instead of hiding the fact that you are moving into an RV, you should let them know. Maybe they can help you move and get organized.