The only way to get away from heat (and humidity) in Mexico is to go up in elevation. Look at Lake Chapala / Ajijic. It's the largest lake in Mexico, about 20 25 miles across if I remember correctly.
It has a huge expat community of Americans, Canadians, Brits, etc. It's about 30 miles south of Guadalajara. Alaska Airlines flies there along with other major airlines so it's easy to come and go from. There are good paved roads over mountains to Maz and PV if you want to go to the coast for a change of scenery for a while.
Guad is the second or third largest city in Mexico so it has everything you would want including a Costco. It has three of Mexico's top medical universities so between there and around the lake you have have good medical resources.
Here's a web board run by the local Caldwell Banker Realtor
Lake Chapal Web Board ClickyThere's not a lot of RV parks there, but I've seen some in person and on the web.
Bill