What evidence is there to suggest America’s self-proclaimed altruism?

No evidence. There is no American altruism and those few altruistic Americans are precisely to whom most other Americans refer when they say "un-American". The US is expressly anti-altruistic and most Americans correctly identify the US Declaration of Independence and US Constitution as specific denunciations of altruism. The US is fundamentally antisocial, culturally and legally, we expect (or even arguably mandate) all individuals to always conduct themselves as rational agents acting in self-interest, we never fight any war for any reason other than our 'national'-rational self-interest and we never offer any pretense of any other motivation. I do not know who ever claimed the US is altruistic but it is either a delusional liberal or an ignorant conservative. I think that is the essential conflict of our current identity crisis, some of us want to transform into an altruistic society while the old school psychopaths want their darwinian blood sport to go on forever.