I have been learning Spanish for some years and would like the opportunity to spend a few weeks in the US at a language school. My husband and I are both retired so can travel at any time but we would also love to visit the National Parks on the west coast of the US i.e. Yellowstone etc. I would really appreciate any information about language schools in the region and wondered about going to San Diego as I know that there is a large Spanish speaking community there and it would be an immersion opportunity.

You would probably not learn the Castillian dialect of Spanish at the schools in San Diego and not in a home-immersion environment. If you are focusing on learning Spanish before traveling to Central and South America, then the dialect of Spanish (mostly Mexican dialect with mixture of some American Indian with Spanish) would be suitable. I took a Spanish refresher class at a college night school in Colorado in 2009, before I traveled to Madrid and Granada, and had difficulty understanding the dialect with simple phrases. That was after I had traveled all over South America without much problem. My mother and grandmother spoke Castillian Spanish fluently, but did notice the difference when attempting to communicate with Spanish-speaking people in California.
Edited by: trekker502

Your talking about 2 separate trips. Do you mean Yosemite? Spanish lessons, go to Guatemala. You won't get a immersion opportunity in SD or LA.
We have noted that Spanish is much more common in El Paso or southTexas than California. It wouldn't be worth it to me to go there GP learn Spanish. You don't have your country listed so it is hard to know what alternatives to suggest. Split the trips and enjoy the parks which require time and planning.
Your idea is perfectly doable. The Spanish speaking communities are large in every Californian city. San Diego might not be your best bet -Los Angeles and surroundings have neighborhoods where almost everybody is Latino (the majority will be Mexican). So you can attend Spanish classes and then walk into a neighborhood store and test your skills. Google Spanish classes in Los Angeles and plenty of options will pop up.
The dialect issue is quite exaggerated IMHO. Yes accents are different from place to place, even within a small country like Spain (the center, Andalucía and the North are quite different) and from one Latin American country to another, but after a short adjustment time your ears get used to it. Save for specific isolated words (i.e. Mexicanisms) the language is the same so it's not really dialectal, it's only different vocabulary same as if you were a Brit in the USA. I'm Argentinian and I had a little trouble understanding Mexicans and Central Americans at first, but I learned quickly.