Natalie, have you and your family ever camped before? For people who have never 'RV'd' before but think they might like it should definitely rent first.
You mentioned 'cars' as in more than one. That will take a nice size trailer and hopefully the cars don't weigh over 10,000 lbs. together including the trailer. A Honda CR-V weighs about 3,500 lbs. To take both cars you will need a diesel coach in the hp range that you mentioned.
What a nice way to teach the kids about this country and they can see everything in person instead of just reading about it.
Good Luck,
MM.
