I found a source of more letters from Benjamin Rush explaining the case for Trinitarian Universalism, in particular letters of his to Universalist guru, Rev. Elhanan Winchester.
Your funeral sermon for Mr. John Wesley does honor to the philanthropy of your universal principles. I admire and honor that great man above any man that has lived since the time of the Apostles: his writings will ere long revive in support of our doctrine---for if Christ died for all, as Mr. Wesley always taught, it will soon appear a necessary consequence that all shall be saved.
-- To ELHANAN WINCHESTER, November 12, 1791.