Well, he was certainly open to the possibility, put it that way.
It was about 1969; apparently he tried to steer her in the direction of Charles but Tricia Nixon wasn't keen on her father's plan.
yes, she had represented the US at Charles investiture as PoW, and when Charles and Anne visited the US Nixon kept pushing her in Chalres direction. I think I read somewhere that he actually preferred the other sister.
Princess Elizabeth and Prince Philip visited the Trumans in 1951.
In Prince Philip, Philip Eade wrote:
In a speech in the Rose Garden, President Harry Truman declared: 'We have many distinguished visitors here in this city, but never have we had such a wonderful couple, that so completely captured the hearts of us all.'