If it were me and my siblings as adults with our own children wanting to go camping in a large enough class A to hold 10 people - with our parents, we would offer to chip in on the expense of buying and maintaining such a large motor-home.
My advice is to get your own smaller travel trailer of class C and have them get their own rigs and get adjoining camp sites. Plan B would be to just leave them at (their) home.
Not sure your going to find what your looking for - price wise or comfort wise, but good luck! Post back with what ultimately happens!