Stop having conversations with people about things that are absolutely none of their business. This isn't only about buying a trailer. This is about your in-laws grooming you to accept that they are going to be in the middle of your relationship, butting in where they don't belong. It's the trailer now. Next it will be when you should have your first child.

These people got their son trained since he was a tot to accept this intrusiveness. So don't expect too much help from him. You've got to role-model for him how to handle this. Simply refuse to talk with these people about this subject. Don't get upset. Don't explain yourself, as to why you don't want a trailer. When you do that, you are allowing these people to control what you talk about.

The way to stop talking about something is to stop talking about it. The way to discourage people from trying to control you is to stop making it easy for them. Privately, tell your boyfriend what you have decided. Then do it. If you need to, put your fingers in your ears and go, "la la la la la, I'm not listening." They will talk about whatever they want, but you don't have to join the conversation.

If you can master this, you will greatly empower yourself. This is just the beginning. These folks are going to be pains in the rear for as long as they live. Start now claiming your right to ignore them. You'll be having to do that a lot on a variety of topics.