First of all, there aren't many US TRPGs. Most seem to be made in Eastern Europe. Secondly, except for freeware games like Battle for Wesnoth and Galaxy Mage, I don't see a lot of evidence of Western TRPGs being inspired by Japanese games. When developers do mention their inspiration, they tend to cite X-COM, Jagged Alliance or Fallout. The origin of X-COM dates back to Rebelstar and Laser Squad, earlier games by the same developer that predate the Japanese TRPGs. Thirdly, it might be more accurate to say that both Eastern and Western TRPGs were partly inspired by the early Ultima games.