Thirty some odd years ago we were going to do the same thing. Back then the rental business was not as well established. After looking for anything that was clean and in good condition we gave up and purchased our first RV, a 28ft Class C. We've enjoyed RV vacations in many rigs since. The point being, if you think this will be an ongoing recreation for the family you may do well to look for a good previously owned entry level diesel RV. It is quite possible that you could purchase a used RV and then resell it for a similar price. That is if you are into private sales of things.
Whatever way you go, enjoy and Happy Trails.