wow, I gotta say this makes me feel sooooo much better. I thought I was the only one who had a family like this.lol I do feel for you. no matter what you do you will never fix the problem with them,they will always be dirty,hard on stuff, and see nothing wrong with it. and when grandpa dies, it will be worse. I don't care how bad your husband wants to keep the peace it is a losing battle 4 both of you and your kids.
the best thing that can happen is you clean it, they wreck it, you clean it, take your trip, they wreck it and your husband finally say's enough is enough. we had the same exact problem with our lake house. end result, we quit going, house got trashed and never cleaned, and finally demo.
the good news is you will see if you like rv'ing or not and your husband will learn you can't worry about everyone else all the time, life is short, enjoy it while you can, we are not guaranteed tomorrow. jm2c.