Discuss how the author has emphasized the "Americanness" of the Otis family?

Wilde uses a variety of techniques to indicate how American the Otis family is. He, for example, uses repetition by consistently referring to Mr. Otis as "the American Minister" or, alternatively, "the United States Minister." Furthermore, he focuses on the American lifestyle and the manner in which they conduct themselves in various aspects of their lives.
Mr. Otis, for instance, tells Lord Canterville:

I come from a modern country, where we have everything that money can buy;

Also, he speaks about:

...all our spry young fellows painting the Old World red, and carrying off your best actors and prima-donnas.

America was seen as the New World, and Mr. Otis's statement alludes to this fact. The Americans were seen as pioneers and entrepreneurs, and Mr. Otis's statement confirms this assumption. America had become a land of opportunity, and all the best talent in Britain left for the United States to seek new opportunities and expand their horizons. His reference to the ghost being placed in a museum or a roadshow further illustrates the spirit of American entrepreneurship.
Wilde contrasts the manner in which the English do things with the American way. Lord Canterville tells Mr. Otis "You are certainly very natural in America," suggesting that the Americans do not assume the snobbish airs and graces so ubiquitous in the English middle and upper classes.
The author also makes particular references to the Otis family heritage to illustrate their Americanness. He mentions, for instance, that,

Mrs. Otis, who, as Miss Lucretia R. Tappan, of West 53rd Street, had been a celebrated New York belle

and that the Otis's had christened their eldest son Washington "in a moment of patriotism." The younger twin boys, similarly, were nicknamed "The Stars and Stripes," alluding to the numerous punishments they received with a switch for their misdemeanors.
Wilde also expresses the political sentiments of the family as wholly American. He states that "They were delightful boys, and with the exception of the worthy Minister the only true republicans of the family."
The references to "Pinkerton's Champion Stain Remover and Paragon Detergent" and "Tammany Rising Sun Lubricator" further relates to entrepreneurial spirit and consumerism, which had become typically American qualities.
The following paragraph probably best sums up how Wilde accentuates the Otis's typically American identity:

The subjects discussed, as I have since learned from Mr. Otis, were merely such as from the ordinary conversation of cultured Americans of the better class, such as the immense superiority of Miss Fanny Davenport over Sara Bernhardt as an actress; the difficulty of obtaining green corn, buckwheat cakes, and hominy, even in the best English houses; the importance of Boston in the development of the world-soul; the advantages of the baggage check system in railway travelling; and the sweetness of the New York accent as compared to the London drawl.