Glyn, welcome to the Van Living Forum
I don't know if you've seen this little Youtube Video, but it tracks the 70 degree weather thru each day of the year across the USA. It can be handy to know about. (it is in the bottom link below)
You may have to stop it every few seconds to be able to understand it as it goes fast. At the top of the video is the month and day so you can plot it on your own road map (with notes) so you can determine where you would want to go for the summer or winter.
Good luck
Where is the normal 70 degree weather