The CEB Study Bible has the following notes by Gordon Matties (professor of biblical and theological studies, Canadian Mennonite University, Winnipeg, Manitoba), on these verses. They suggest much deeper significance.
5:15 Now I know … no God anywhere on earth except in Israel: Although 2 Kings 5:8 anticipates Naaman's knowledge of Elisha's prophetic role, here readers discover the heart of the matter. Naaman's confession echoes Solomon's confession at the beginning of his prayer at the dedication of the temple (1 Kgs 8:23). Naaman is a textbook example of the foreigner (translated "immigrant" in 1 Kgs 8:41) who "comes form a distant country because of your reputation." The hope, prays Solomon, is "so that all the people of the earth may know your reputation and revere you" (1 Kgs 8:43). Similarly, Solomon's blessing on the people also includes the hope "that all the earth's peoples may know that the LORD is God. There is no other God!" (1 Kgs 8:60). Naaman has demonstrated the fulfillment (perhaps as a representative of all foreigners) of Solomon's hopes. 2 Kings begins with Ahaziah going to inquire of the foreign god. This is a problem because, as Elijah puts it, "Is it because there's no God in Israel" (2 Kgs 1:3) that Ahaziah is consulting Baal-zebub? Naaman puts that question to rest. Not only is the LORD (Yahweh) actually Israel's God, but the LORD is the only God on earth. Perhaps this is the grandest confessional statement in the books of Kings, and that from the mouth of an outsider.
5:17 two mule loads of earth: Although Elisha refuses payment, Naaman asks for earth. Some suggest that he wishes to make a mud-brick altar when he arrives at home (cf. Exod 20:24-25). Others think the earth represents the sacred land of Israel, the land of Israel's God; and the Lord's power is limited to that territory. The more important point is that Naaman's worship won't be directed toward any other gods except the LORD.
5:18 may the LORD forgive: Naaman's request echoes Solomon's prayer at the dedication of the temple (1 Kgs 8:30, 34, 36, 39, 50). He represents the foreigner who prays toward the temple (see note on 2 Kgs 5:15). Rimmon: another name for the Aramean storm god Hadad (cf. 1 Kgs 16:31-33; Zech 12:11).