Here is where I have a very difference of opinion. The question is, how often will you have kids with you? Every trip, once a year?
The bottom line is, you should select the camper that fits you and your spouse, so you are comfortable, you can utilize the space, and enjoy the comforts .... JUST the 2 of you! You have raised your kids, and more than not, you will probably find yourself camping, just the 2 of you. So get a GREAT camper that works for the 2 of you and forget about the kids and grand kids.
After you select the camper that fits you, the kids will fit pretty much anywhere. They may even perter a tent to the inside of the camper too.
Now, if you have full time care of one of the children, that is different, BUT you cannot count on the extended family being with you. Get the camper that fits just the 2 of you. The rest will take care of itself when the time comes.
Besides, I think if kids want their kids to experience the camping experience, they should get their own camper. Don't get dumped on and don't be guilted into taking the grand kids so the parents can have "their time". You and your spouse need "your time" now! It's time to rekindle the relationship between you two. This is a perfect way to do it.