I lived in NM for many years, most of it was in Silver City. It was over 6,000 feet elevation and mountain tops near by were over 9,000 ft. Natural air conditioning. Like you, only about 3 weeks a year when AC was needed and swamp coolers were more than adequate. And very cheap to operate.
I own a home at the farm that I plan to move back into in the spring. When I complete upgrades. When it was built there were no swamp coolers as such. They had window pads on some windows and an exhaust fan in the center of the house that drew air from the windows unto the space in the attic area and it exhausted in the attic vents. Very efficient. And there was NO EPA or Department of Energy. When my ancestors moved to this part of Texas in 1889, (the county was cut up in 1885) there were no roads (only trails), no electric power, little water and it was a very harsh place to settle. A number of branches of that hardy group still live here.
They Department of Energy and EPA can pound sand.
Swamp coolers are only effective in relatively dry humidity. They're very effective over most of the Rocky Mountain region.
Anywhere roughly east of the Mississippi, it's far too humid; you need an AC unit to remove the moisture before you can realize any cooling.
What part of TX?