You're definitely overdue for a roadtrip!
I agree that your kids are old enough to be part of the planning process. They're likely to enjoy it more if they've had input into it. I suggest getting a good size map or atlas and letting everybody pour over it. They can mark the places they want to go. Maybe everybody could use a different color highlighter? Places that are picked by several different people should definitely make your list.
Is your goal of getting to San Diego and LA firm? Since you haven't done a roadtrip before, I'm wondering if you might have more fun if you make it flexible. Just hit the road and stop at the first place you all want to see. Enjoy it for as short a time, or as long of a time, as you all want. Then go to the next place. Stop for the night when you're ready. Heck, does it matter if you only get to New Mexico or Arizona and not make it all the way? If you're having fun, then I think not. Just do this until you get to about day 5 or 6 and then return home via a different route, stopping along the way at the fun things you've picked out on those roads.
I'm thinking this might be a better way to do your first family roadtrip. Right now, you don't know how much any of you will enjoy being in the car for hours on end to make it to SD/LA and home again. If you try to push it too much, you might all be exhausted and sick of being in the car and get turned off to roadtrips. That would be really sad because roadtrips can be a real hoot. This gives you a way to assess what your family's interests and limits are. You could always plan another trip to SD/LA next year if you determine, after this first trip, that that type of driving distance will work for you and your family.
As for the RV, imho, I really think you'd want to rent an RV for a shorter weekend trip first to see if you like it. Driving one is quite different than driving a car, even if it's a big Suburban. And the process to hook 'em up, level 'em out, etc. takes awhile. Especially if it's something new to you and you aren't sure of what you're doing. In fact, I think most folks would agree that driving them is a bit more fatigueing so you're likely to cover less miles if you drive one. And by the time you pay the rental fee, campground fees, and for the extra gas, I would bet that you'll actually find that it will cost you more than using your Suburban and staying in motels. Of course, if you're interested in camping, you should have plenty of room in your rig to pack a tent and sleeping bags. But, if that's not you're style, I still think you'd do better to stay in motels.
Another option that could be fun is staying in campgrounds but renting a camping cabin. Most KOAs have these and increasing numbers of other nicer campgrounds have them, too. They don't usually cost much more than an RV camping site with hook-ups. Something to consider anyway.
We used to have a truck/camper combo and then, when the kids got too big to sleep on the bed for them, we pulled a small trailer with our truck. We had a lot of fun in it, don't get me wrong. But we typically never took it on trips farther than about 600-700 miles away from home. We found that anything above that made the extra fuel costs so expensive that we could do better by taking our car and moteling it.
If you want to avoid eating at restaurants each meal, you could still do picnics out of a cooler. You could even bring a campstove and some chairs.
