Perhaps, my estimations were a little understated. My wife and I routinely travel in the Rockies, so anything under 6-7% grade and a couple of miles long doesn't seem too bad to me.
The east and west approaches to Tehachapi both have a couple of grades where heavily loaded semi's get bogged down for a couple of miles. It seems like nearly all RVs do OK on these stretches, unless they get caught behind some slow semi's and can't pass due to traffic. Hwy-58 through Tehachapi is not nearly as long or steep as Cajon Pass between Victorville and San Bernardino, and I don't think an RV in good running condition would have any difficulty with these grades.
If you want to bypass LA and drop down to Joshua Tree National Park and I-10, you can take Hwy-58 east to Kramer Junction (about 5 miles east of Boron), then turn south on Hwy-395 to Adelanto.
In Adelanto, turn east on Air Expressway (a.k.a., Air Base Road), which runs between the old southern entrance to George Air Force Base (now the Southern California International Airport) and a prison. On the east side of the airport, turn right (southwest) onto National Trails Highway (a.k.a., Historic Route 66), which runs along the northern outskirts of Victorville and on east through Apple Valley.
At the junction of I-15, this surface street turns into D-Street/Hwy-18. In town, near the train station, on the south side, you'll find the California Route 66 Museum, which contains a lot of interesting information about the old Route 66, George AFB, Roy Rogers and Victorville. A couple of years ago, they had an old teardrop trailer on display; not sure what's there now.
Celebrating the Historic Mother Road… California Route 66 Museum, Victorville, CA
Continue east on Hwy-18 through Apple Valley (home of Roy Rogers and Dale Evans, and former location of the Roy Rogers Museum and Trigger) to Lucerne Valley. Then, where Hwy-18 cuts off to the southeast, do NOT turn. Continue due east on this road, which turns into Hwy-247.
Continue east and south on Hwy-247 (it wanders quite a bit) to Yucca Valley. At the junction of Hwy-62, turn east and continue on the 29 Palms Highway (Hwy-62) about 4 miles to Park Boulevard.
Turn right (south) onto Park Boulevard, which goes through a residential area for a few blocks (watch for the Joshua Tree National Park Visitor Center on the right/west side of the street); and continue south on Park Boulevard, which eventually ends up at the north entrance to Joshua Tree National Park.
Camping - Joshua Tree National Park
Note: Don't forget your National Parks Annual Pass or Senior Pass (if applicable), which allows free admission to the park. Also, the Senior Pass gets a half-off discount on camping in Joshua Tree National Park and other National Park, National Forest, BLM, Army Corps of Engineers and other National campgrounds.
National Parks and Public Lands Pass Series - Joshua Tree National Park
There are several campgrounds in Joshua Tree, but the ones in the north end of the park fill up fast in the early afternoon. If you get there near dusk, take the first camp site you find; because it won't be there if you go back. If all of these sites are taken, there are plenty more near the southern end of the park at the Cottonwood Campground, but it's about 20+ miles of winding roads (in the dark) at 35 mph. There's another visitor center, fresh water and a dump station at Cottonwood, and I-10 is only a few more miles further south. The junction with I-10 is about 5 miles west of Chiriaco Summit and the General Patton Memorial Museum.
General Patton Memorial Museum
If you continue east on I-10 through Phoenix and Tucson, you'll go near Kartchner Caverns near Benson, AZ. Then, in Las Cruces, you can decide whether to head east towards Carlsbad Caverns National Park or back north to I-40. There are lots of interesting sites and campgrounds on the roads north through New Mexico, including:
* White Sands National Monument
* Very Large Array (VLA) radio telescope antennas west of Socorro, featured in the movies 2010, Contact, Terminator Salvation, and Transformers: Dark Side of the Moon
* Trinity Site (atomic bomb test site, open for viewing the first Saturday in April and October)
* New Mexico Museum of Space History in Alamogordo
* Balloon Museum and Albuquerque Balloon Fiesta (in October)
* Valley of Fires State Park near Carrizozo (interesting boardwalk over volcanic flows, with great BLM campground, eligible for Senior Pass discount). Clean showers, fresh water and dump station.
Lots to see and do in Arizona, too. Check this site for other posts on Arizona and New Mexico attractions.