I agree with twogypsies. I think you will be fine. There may be some curves where you may have to hug or go over the fog line but I can't remember it being an exceptionally curvy or narrow road.
The KPRA 30' advisory will still allow buses and motorhomes under 40' to use that roadway. Attached below is the CalTrans District 9 map for motorhomes and buses.
The CHP officer I asked about toads some years ago told me that motorhomes towing a car are not considered to have additional length as the car will track with the vehicle towing it. I'm not sure if this was his personal interpretation or if it's actually addressed in writing somewhere. If an interpretation, it could be a bit "iffy" legally, I suppose.
Pertaining to I-40 eastbound from Barstow, I now think I know where you're talking about. I believe you're referring to Mountain Springs Summit just west of Needles. When you said "Mojave" I just assumed that you were referring to Tehachapi Pass between Tehachapi and Mojave on CA58. I never considered Mountain Springs Summit that steep of a grade as I'm originally from the Pacific Northwest but I suppose with the temperatures in the summer (could be 120 degrees or more at times in that area; one of the hottest places in the country), it could cause some overheating with some vehicles.