No matter whose mother in law it is, she is the other's mother. And vice versa.
Only you know your family's social dynamics. The can't stop talking would be a problem for me (distraction while driving) but if you can't understand what she is saying, that could compensate.
How does having mom around affect your wife's behavior. My wife's mom got on her nerves as much as she got on mine. We both got along better with my mother, although she was not an easy person either.
Have I done it? From a grandchild's perspective, four week trip from Detroit to western edge of Montana and back to settle some family business. Mom, Dad, my Dad's Mom and Dad (in their 80s) with six of us kids, age 15 1/2 down to four, four boys and two girls, teen and tot. Packed into a 1960s station wagon, pulling a 16-ft TT that managed to sleep all of us when necessary.
By the time we got back home, we were still all talking to each other, and there were no major violent incidents.
Three related people in a large motorhome seems really easy, but there are other parts of my dad's family that couldn't get through a holiday dinner without a fight, or at least some serious sniping. Not even my dad's funeral stayed peaceful, and only his two oldest brothers remained around to show up. Come to think of it, when my wife's father died, things got nasty between her mother and her dad's sister, behind the back stuff but those two never got along, and I would take neither of them with me on any trip; taking one out to eat is enough of an ordeal.
So it all depends on the dynamics of the people involved, and I can't help you with folks I don't know.