We just did a 4 day trip in a converted sprinter van with my wife, my 26 year old and my 22 year old son (who is more like a 150 pound toddler so he didn't get a vote). It went great, we all had a grand time and none of us got into a fight. It can be done.
Agree. That's what I mean, I went camping with my in-laws on multiple occasions in the past when I was around that age (granted, it is a while back...). Or even went on vacation with my spouse and my parents for some years despite having my complete own life. It is not that unusual.
However, it does occur that there is more to that here. I also don't think they went camping just for fun.