If I were doing that trip, I'd drift south at the beginning of the trip and head north as summer progresses. Remember, that most of California is a desert. Think Death Valley.
Taking I-5 up into Oregon and Western Washington (where I was born) is interesting and it's cooler then down south. Taking 2 east at the end of summer can be fairly comfortable...temp and weatherwise. US2 in North Dakota is sparse people wise but there are some fun places to stop along the way. Several bars with food have little RV parks out back.
Dipping your toes into the lakes is kinda fun too then because the water is fairly warm at the end of summer.