Do NOT take 74/79/371 from Palm Desert area!! It is steep, winding, and not recommended for RV's! Main problem area is on 74, 10 miles south of Palm Desert. Take I-10 to 60 to 215 south to Temecula. All easy driving. We drive this route from Phoenix several times a year.
60 does have a very short mountain pass to go through, maybe 2 or 3 miles long east of Moreno. Not steep but winds just a bit. All 4 lane. Stay in right lane as left lane is somewhat narrow with cement center divider.
If you are in the Phoenix east valley or south towards Ahwatukee area it may make more sense to take 347 (Maricopa Rd) to I-8 to I-15. That would avoid driving across metro Phoenix. I-8 has a long grade west of El Centro and traffic can be an issue on I-15 during rush hour.
2014 Winnebago 26FWRKS 5th Wheel
2007.5 Dodge 2500 6.7L Diesel
2004 Dodge Durango Hemi 3.55 (Used to tow TT)