I think you need to start with your DH. First, I am 100% sympathetic with the desire to go to a grandparents at Christmas. I did it every year from birth till my parents divorce when I was 14. And I have nothing but fond memories of the days and nights spent with aunts, uncles, cousins, etc. And, if I had chilfren, I'd want them to have that too. And, while I agree with those who say they don't want to travel at all on the holidays, I also think it's not the solution for everyone, and is clearly not your ideal solution. So, your DH has a decision to make. And so do you. Is it important enough to you to maintain the tradition that you're willing to have the uncomfortable conversation to make it happen. It is not feasible to keep going along as you have been and it's unhealthy and potentially dangerous. And you're not going to get your ILs to change by wishing for it.
So, I think your DH needs to have an open and honest conversation with his family about why you won't be visiting their home in its current condition. No side-stepping, no half-truths. Plain facts. Because if it's really about the dirt, then you can't ask them to solve the clutter and hope the dirt follows or vice versa. You need to figure out exactly what it would take to make you comfortable and give them that information. Nothing else is going to work. And it will either open their eyes or it won't but at least you've laid all your cards on the table and you let the chips fall where they may.