Just a few more thoughts about your travels in California. A lot of people seem to use Yuma for their overnight stop out this way but if it would work out for you there are a couple nice campgrounds in El Centro, too. And it gets you 60 miles farther up the road. But then there is a bit of "magic" to the historic nature of Yuma.
Heading out of Temecula, You might be tempted to take 74 between Lake Elsinore and San Juan Capistrano over to I-5. I agree that you don't want to go that route. It's a heavily traveled mountain road with some tight curves, few places to pull over, and nasty drop offs down mountain sides if anything goes wrong. Just bite the bullet and take the 91 farther north. It's probably only another 10 miles.
Instead of taking 78 from Salton Sea over towards Julian maybe think about going 12 miles farther north to county road S22. It takes you west over to Borrego Springs - - which is a really nice little village with a great up-scale RV campground :
The Springs at Borrego. A beautiful place and they have a 9-hole golf course all around you to either look at or maybe get out and bat a few balls around. Be aware though that the golf course (not the campground) is closed down late in summer for maintenance and to allow for some recovery time from the really dry desert heat.
Continuing on from Borrego Springs by the most direct route requires that you stay on S22 - but that involves an 8 percent climb up the Montazuma Grade for over 10 miles. It's a wide road with mostly gentle curves and some great views back over Borrego Valley. You climb from about 600 feet at Borrego up to over 4,000 feet at the top of the grade near Ranchita.
An alternative to climbing that grade is to take county road S3 south out of town across a low ridge down to 78 - - which was the original road you had thought about taking. It's a gentle climb going south but the drop down onto 78 is a little steep and has some tight turns. But it's only a mile or two on that downgrade. Go east on 78 at the bottom of the grade and then take a right at Scissors Crossing up S2 all the way to 79 near Warner Springs.
Or you could think about continue east on 78 for a climb up the Banner Grade into Julian. The Banner grade has some tight curves and the road climbs at 6 percent for 5 miles. But it should not be a problem for a motorhome. Be aware that California restricts a lot of these mountain roads to motorcoaches not over 40 foot foot in length. Key words "not over." Maximum overall length towing is 65 foot.
Cal Trans Motorcoach Map.Another route to consider. Continue west 25 miles past El Centro to Ocotillo where county road S2 begins. Take S2 north up through some really nice desert country with a range of mountains immediately on your left. You end up at Scissors Crossing on route 78 where you would just continue on S2 up to 79 near Warner Springs. It's a 50 mile lonely drive up to Scissors Crossing. Till the end of May the Aqua Caliente San Diego County Campground 20 miles up S2 is open (30/50 amp). And four miles farther north is the Vallecito Stage Coach Station county campground. They have water faucets scattered around there but no power, dump, etc. You can catch an ATT cell tower there most days for some strange reason.
But there is not much else along S2 untill up near Scissors Crossing where two small, rustic private campgrounds are available (Stagecoach Trails & Butterfield Ranch). .For some reason they are listed under Julian which is 35 miles away and up in the mountains a couple thousand feet above them. These are 30 amp campgrounds which in the extreme heat to be expected in that area might not be adequate unless perhaps you were arriving late in the day and temps were not too high. These are a just place to park basically and overpriced. I only mention them because campgrounds are few and far between along your route in this area.
Interstate 8 to San Diego climbs up out of the desert just beyond Ocotillo with a 9 mile grade of 6 percent. It's somewhat of a tough climb and you are probably starting with temperatures of well over 100 degrees during a typical summer. Once up on top of the mountains there are a few other up and down grades across the mountains but with normal care they should not be of any concern. Just watch the downhill speed. The decent into San Diego from the Pine Valley area is very long but there are occasional places where it levels out for a bit. Nothing to worry about at all.
Just one other comment. About the only route between the Coastal Cities in Southern California and places farther east without having to use any mountain grades is Interstate 10 past Beaumont and Banning in the Palm Springs area. (Though S2 between 78 and 79 is a very minimal grade but in a very rural area.)
Hope this gives you some ideas for your travels out this way.
