I'm afraid this is a little rambling, but I couldn't think of any way to condense it.

My wife and I live about 2 hours away from our families right now, but for the past couple months she has been traveling weekly to [Hometown] for some medical treatment. She and our kids are usually gone for 3 days/week. Whenever she goes to Hometown, she always stays with my folks: they have extra room in the house, it's clean and quiet, and easy for the kids to get the rest they need. This has been a perpetual source of grief for my FIL, who hates that my wife is being "influenced" by my parents and wants her to stay with his family, who live a ways outside of Hometown, have no extra room, and stay up until 1 or 2 AM nightly. (She goes to visit them all the time when she's in Hometown, but always leaves at bedtime.)

FIL has made my wife feel super guilty about not staying with him, but for the last 2 years she's managed to avoid it by pointing out the conveniences of staying with my folks instead. She has never told him that she just doesn't want to stay with him, though. So, this week, he announced a great surprise: he purchased an RV, so that she and the kids have somewhere quiet to sleep, and can keep bedtimes, and all that other stuff.

I don't want her and the kids staying out there, and I know she doesn't really want to, either. However, now all her old excuses don't work because FIL has "fixed" them (of course without asking). When she told me about it, I said it sucked that her dad put her in that position, but that I didn't want her or the kids staying out there.

Long conversation, with me trying to use fogging and broken record the whole time. She kept pushing me for reasons why they shouldn't stay out there, and I kept doing my best to avoid giving reasons and repeating that I didn't want them to stay in an RV. This led to, "You won't even talk to me! You're just telling me what to do!" or something like that. (Me, sample answer: "I'm not telling you what to do, and we can definitely talk, I just don't want you and the kids staying out there.") The longer it goes, the more defensive she becomes.

She wants to have another "conversation" about this later this weekend, and I'm not sure how to handle it. I really feel like she's a proxy for my FIL: I don't really have to give her reasons, but I have to give her reasons that would satisfy him. My primary reasons are (1) I don't trust FIL's motivations, and (2) we've spent a long time establishing a precedent that wife and kids do not stay at his place, and giving in would ruin that. How should I handle this conversation? More broken record? I don't even want to compromise (e.g., staying out there every other week) because that would be a slippery slope that would give FIL even more control. Any suggestions?

EDIT:

I should have mentioned that wife does not at all want me to call FIL and tell him no. She's afraid that if I do, he'll hate me more than he already does. This doesn't bother me. But, what worries me is that whenever I piss him off, he takes it out on my wife and makes her feel like a guilty pile of shit who has betrayed her real family. So, in order to spare her his crazy bullshit, I've tended to avoid confronting him about things. Hence, the proxy war.