Hello Kristjan Einarsson,
Actually, you don't need a converter if you use the CheckState property of your RadTreeViewitem which is of type System.Windows.Automation.ToggleState. Please examine the approach in the attached solution and let us know if it helps you.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items