This couple has 12 children and are living fulltime in a 36' motorhome. Here is the link to their blog Kellogg Show Â» Living in an RV with Twelve Kids
It can be done if the family is up to it. It's not for everyone and probably only for a few. However, if you really want to do it, you will make it work.
Here is another link Families on the Road
that has links to many families on the road.
Do you research as there are many families in rv's fulltiming. Make sure this lifestyle will work for your family before you sell your house.
When a couple takes up this lifestyle it is important that both really want to do it; otherwise it won't work. This will also be true for a family since there are more people. If your 12 year old isn't on board; it could be more difficult to make it work.