In 2012, we left Texas in May, then June was in CA visiting. We spent July and August on the California coast with temperatures hovering between 65 and 70 degrees. We beat the Texas heat. Our last stop in California was Four Seasons RV Resort on the Rogue River. We took the 104 mile round trip ride in a jet boat and never regretted it. We then headed to Sutherland, OR. There is a wonderful Escapee's park their with all lots 55 feet wide by 80 feet long. No reservations, first come first serve and we called ahead and there was room at $118 a week (Oh oh! Let the at cat out of the bag.) Then up to Washington, across to Missoula (Jim & Mary's - nice) then to Columbia Falls, MT at the RV park there for our excursion into Glacier NP. Several stops later across I-90 we stopped at Ellsworth AFB and after that headed South, back to Texas.
We were in a 40 ft DP pulling a Saturn Vue. You can search on tips and tricks for ascending and descending hills. I-90 is all doable, just keep right and let the others pass. Always, always use lower gears going downhill and use the brakes sparingly. We did 4600+ miles and enjoyed every moment. (Well, sans one when the awning billowed out, but that's another story.)