I can't imagine how nice it would be to visit the Grand Canyon in 1941 ... only a handful of tourists at the overlooks and quiet nights in the campground.
And the campgrounds were free (up until 1965)
Before WWII most people didn't have decent automobiles and access to some of the national parks was by train. Both Grand Canyon and Yosemite were serviced by tourist train--the Grand Canyon train runs to this day but Yosemite's is long gone. Many parks had post offices, leading to park oriented postal history. Is it coincidence that the earliest postmarks in the postal history article below coincide with the tourist railroad's inauguration?
A few links for your Holiday perusal--Happy Fourth!
Grand Canyon Train: https://www.thetrain.com/the-train/schedule-route/
Yosemite Train: https://www.nps.gov/yose/blogs/trac...railroad.htm
Yosemite Postal History: http://nostalgia.esmartkid.com/yosemitepostal.html