We have always used the "Mountain Directory West for Truckers & RVers" with 100% success.
For Hwy 74 it states: "Between Lake Elsinore & San Juan Capistrano, CA - tractors-semis over 30' kingpin to rear axle not advised. There is a 5 mile 6-7% eastbound descent into Lake Elsinore with a T intersection and stop sign at the bottom of the grade. This road is cut into the side of the mountain so there are long steep drop-offs and dramatic views of Lake Elsinore far below. The descent on the west side is spread over many miles so there are short descents with mild grade in between. The entire road is rough, narrow and winding with heavy traffic including many trucks."
This book is geared for truckers who are bigger than RVs. So if truckers are not advised, RVers are not either.
If you travel the western roads it's worthwhile to buy the book. There's also an Eastern version. We love mountain driving and we always referred to this book before taking off for the unknown. We always knew what to expect. No surprises. We would not take the 40' motorhome on this road.
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel