RoutedViewHost is now in ReactiveUI.Xaml.dll, which is in the ReactiveUI-Platforms NuGet package. Since you haven't specified which major version of RxUI you're using, it's more difficult to help.
In ReactiveUI 4.x and below, this is in an assembly called ReactiveUI.Routing.dll, which is in the ReactiveUI-Xaml NuGet package.