One of the joys—and challenges—of mobile living is that we can chase ideal weather instead of putting up with the climate in one spot. That's one reason I'm in transit today, playing climate Goldilocks. Too cold... too hot... just right.
If you don't mind putting on a lot of miles, you might follow meteorologist Brian Brettschneider's route that should, in theory, keep you in lovely 70°F weather all year. He has a shorter one in case you don't want to drive all the way to Alaska. There's also a nifty animation showing the various 70° sweet spots each day.
However, daytime highs are only part of the story for me. Humidity, precipitation and nighttime lows also matter. I'd take a clear, dry 85° over a foggy 70° any day—though I did a lot of the latter this summer just to avoid 100+ temperatures. You might want to try Brettschneider's road trip before continued climate change makes his data and calculations irrelevant.