I agree with the renting idea. How your family moves around in an RV will be different than how someone else's family moves around in an RV. Renting the different types of RVs will let you try them on for size.
Also making lists. Must Have. Really Really Want. Would Like. Don't Want. Absolutely No Way.
Things to think about: carseats; ability for toddlers to potentially move around while on the highway (some toddlers have been known to undo their car restraints); how long you'll spend in one place; sleeping arrangements; bathroom stops while on the road (I was a child who ALWAYS had to go 30 minutes after we left the house - no matter if I did before we left); sick kids - simple stuff like how to rearrange sleeping; what type of places will you stay at (RV Parks, National/State Parks, National Forests, etc.); what types of hookups will those offer; etc.
I'm sure others can add to this list quite easily.