Think about fuel! Remember gas costs more on the road and in some states.
Think about fuel but not about prices. My parents were driving from Texas to Vegas and nearly ran out of fuel. Going uphill with the airco on max uses up a lot of gas, and they weren't even driving an RV
so plan fuel stops conservatively. They ended up filling up somewhere in the mountains where fuel usually costs over 50cts/gal extra. The point is not to get distracted by fuel prices, they will be the least of your problems.
Be prepared for some bad weather and extremes, I've seen close to freezing torrential rains in Flagstaff, Arizona, just one day after going through Death Valley @120F...bring necessary supplies, attire and water. Make sure you now about local laws regarding camping, food in the outdoors (bears) and prohibited activities (fire) especially in the nat. parks. If you can, bring a GPS that functions in the car as a planner, but also on foot, when you're enjoying the parks.
Also note I've had to replace tyres in both Death Valley and along route 95, always at the most inopportune moments, either at night or in 120 degree heat. So come equipped with proper gear, tools and repair kits or call AAA (ever tried to jack up an RV?)
Check with the AAA and local (California) DMV if any special requirements need to be met while driving a rented RV (IE are the younger members of your family insured to drive, in case of emergency?) California laws pertaining to out-of-state vehicles and licenses may be a bit stricter than you're used to.
Planning the trip with a visit to San Diego in the middle is probably a great idea. I suggest you plan at least a couple of days in SD for resting, your family is going to be pretty stressed from two weeks of travel and you need to be rested for the two weeks back. SD is the perfect place to recuperate. (La Jolla cove, Coronado, Sea World & wildlife park/zoo, the beaches at Carlsbad/ Del Mar/ Cardiff, Old Town, Balboa park, gaslight district, etc.)
Lastly, I believe you and your family all play music together so I would try to plan some of that on the road, in whatever way you can, in order to relieve tension, which you will not be able to avoid
At times you may need to remind yourself that you're supposed to have fun on the trip