My 2 cents which is worth a lot less.
How many days do you want to stay in that area.
Three weeks from Columbus, Ohio to the Santa Barbara, CA area and back is at least 8 days driving - long and hard. Closer to 10 or 12 pulling an RV. 4,680 miles down I-45 from St Louis and I-40 from OKC to Barstow, CA, over the pass and along the coast.
Most folks can do 500 miles a day in a car with no problem, sometimes 600. 400-500 is a long hard day in an RV, especially if you are not used to it, or plan to drive daily. An hour for setup, an hour for take down in the morning.
Will you be doing all the driving or will the wife be comfortable driving the rig? After four days, you are going to be dead tired.
You are also going to want to run in the 55-65 mph range. Going to 75 will increase your fuel usage as much as 25%.
I use Microsoft Streets & Trips to plan general routes.
Going to Yellowstone and staying four days, then to CA and stay four days, and back to Ohio is near 5,200 miles. At least $1,700 in fuel at $3.50 per gallon if you can get 11 mpg on the highway. It leaves you four days leeway in-case something looks interesting, or there are problems, or you just need an extra day of rest.
That said - don't let it put you off completely. I would suggest your first big adventure run no farther than Yellowstone and back. Save the CA trip for another year - or vice versa.
Three weeks seems like a long trip, but you will be amazed how quickly the time can pass.
I plan my trips for 350-400 miles per day max. I also plan a two or three day stop every week, usually more often.
We are going to Hartford CT from Dallas in June - we will stop at a COE park for one night in Arkansas - a COE park for two nights near Nashville - a COE park in Pennsylvania for one night - and my brother-in-laws home for a week outside Hartford.
Reservations - In California near Goleta and / or Yellowstone - make your choices soon, and make the reservations soon.
I would ask in the Campgrounds section for info for parks in both areas.
Another good resource is http://www.rvparkreviews.com/ with the understanding that complainers post more than people happy with the parks.
