My understanding is that the laws that define who can be labeled exempt from overtime are federal and not state. I am not knowledgable on tribal laws however so don't know if they apply here as well. Are there any tribal labor laws that do apply to you or is there any government entity that you can consult?
I did a quick Google search with the terms - tribal labor laws, overtime, casino
This is the link to one of the articles that came up and it may give you something to go on.
www.indiancountry.com/content.cfm?
From the articles I saw it didn't seem clear that federal overtime laws applied but you might either call or email someone at the US labor department wage and hour division to confirm.
Is your supervisor looking to pay you more money for becoming salaried or is this just an opportunity to cut the pay?
Good luck