***** NOT A TEACHER *****
Hello, Ademoglu:
1. I agree that "All she saw were the same white walls of the hospital" sounds the most natural.
2. There may still be a few very strict teachers in 2014, however, who feel that the "correct" answer is "was."
a. One expert* feels that "all" = "the one thing." Therefore he gives examples like these:
i. "All that came to him ... was voices."
ii. "Well, all you need is a cup of molasses, 2 teaspoons of vinegar, 3/4 cup of sugar."
iii. "All that remained to be done was the small jobs of touching up and adding refinements here and there."
b. Another expert ** says this: "When all is the subject of the verb to be followed by a plural complement, the linking verb is expressed in the singular."
He gives this example: "All I saw was fields."
c. But even in the 1930s, yet another expert *** admitted that many people were speaking like this: "All that we found of the deer were the ragged hide, some patches of hair, cracked bones, and two long ears."
My respectful advice:
1. If you want to speak natural-sounding English, you should consider following the advice given by the two teachers in this thread.
2. Hopefully, no teacher will have this kind of question on a test, for there are, in the opinion of many people, two "correct" answers.
3. I think that it would take a lot of courage to use the singular in your sentence, for many native speakers would probably tell you that you have made a "mistake."
James
* Wilson Follett, Modern American Usage (1980). Some people consider Mr. Follett much too strict (old-fashioned!).
** R.W. Burchfield, New Fowler's Modern English Usage (1996). Doctor Burchfield was a distinguished scholar, whose positions included Chief Editor of the Oxford English Dictionaries. Nevertheless, some people do not agree with some of his opinions.
*** George Oliver Curme, A Grammar of the English Language (1931). Some people criticize him for being old-fashioned, but he was modern enough even in 1931 to observe that many people were not observing the "rule."
