I have a mostly invisible disability (2 of them, actually, as life has continued its journey...).
I always request a wheel chair, and have never had a problem requiring an explanation. I've been asked appropriate things such as "can I climb stairs?", "can I walk down the airplane aisle myself", etc.
I cannot walk long distances and especially cannot stand in place for very long, so I just explain that, and have never had anyone question it, not in the USA/Canada, several countries in Western Europe, and in Japan or Hong Kong.
However, I do sometimes feel that some other passengers are staring daggers because they don't "see anything". In that respect, it was sort of easier for a few years when I had to wear one of those medical walking boots. (Back then, it always generated a huge delay at security, because they had to do the slower chemical swabs, which seemed to be triggered by a lot of false positives, leading to more swabbing. The new techniques seem far more robust/accurate, which is good for many reasons!).
For a few meds, we do carry physician letters and original scripts, but... we've never been asked for any such thing.
GC