We were there in April and May (6 years ago now) and I wouldn't say it was 'peak' season but most areas we visited were busy. The weather was good but it did vary a lot depending on where we were. We spent time in Canada too but here are the major spots we visited in the States and what they were like.

We flew into LA and the weather was mild - cargos or shorts and t-shirts most of the time. Vegas was pretty warm but the Grand Canyon, just the day before we arrived in Vegas, was freezing!! It actually snowed while we were there and would have been below 10 degrees for the entire day we spent there.

San Fran was really cold too, low teens if that.

NYC was fresh, pants and cardy/light jacket most of the time while Miami was hot. We lived in bathers

This was all in a 6 week period from the very end of March to mid May.

We were in Vegas again 2.5 years ago (Oct 2008) and it was warm during the day but being the desert, cool at night. We stopped at LA on the way home and the weather was nice there too, quite mild.