Actually, he does note that is based on a true story early in the letter. I posted only a portion of his comments, as the letter is rather long. You can read it by clicking on the link.
It would be interesting to read the original book by the real-life governess, Maria von Trapp, to see just how far Hollywood departed from reality in the film starring Julie Andrews and Christopher Plummer. However, we need not know the original to see what Hollywood has done!
In fact, the Bishop might have made a much stronger case by noting how the film did depart from reality&mdas. I haven't read Family on Wheels, but have read Maria (the prequel) a few years back, and that, along with the Bishop's comments, was enough to make me dig around on the net and find the information that I tried to link to in post 18 www.archives.gov/publications/prologue/2005/winter/von-trapps.html .
As it stands, the film is good when compared with what passes for entertainment now days, but the Bishop does seem to have valid points in that history is altered for the sake of promoting an agenda, and the more one is aware of the history, the more the agenda is apparent. The Catholic children's author Hilda van Stockum, who was a friend of the Von Trapps, walked out of the Sound of Music because of the way it had altered things (I have this from one of Mrs. van Stockum’s granddaughters). In fairness, I ought to add, that, while friends, Mrs. van Stockum found Mrs. Von Trapp to be rather cantankerous (the link also notes the trait) and apparently found Julie Andrews to be the thing hardest to stomach.
Another instance of Hollywood making what passes for a decent movie that actually tilts social mores left is Yours, Mine, and Ours—which I really liked in the original form until I read the book upon which it is based, Who Gets the Last Drumstick? Thanks to this evening, I suspect that I will read Family on Wheels and never want to see the Sound of Music again.