The term Priest as used in Genesis does not necessarily mean a Religious Leader, but in fact may have only designated the leader of a tribe and etc. the following is an excerpt from Strong’s dictionary.
Active participle of H3547; literally one officiating, a priest; also (by courtesy) an acting priest (although a layman): - chief ruler, X own, priest, prince, principal officer.
Jethro most probably, as the priest of Midian, was the comparative to a Mayor in our modern day.
As far as his knowledge of the Hebrew God, since Moses was his son in law, Jethro would as any father have queried Moses on many thing before allowing him to marry his daughter, especially since he was a leader to his people. It is also possible that Moses had convinced him to worship the Hebrew God.
Moses as an enlightened believer, gave a convincing description of the origin of all things; as in Chapter one of Genesis. We are not aware of any other explanations of Creation, prevalent at the time of Moses. It must be remembered that God spoke directly with Moses.