Yes, you are right, this problem is due to floating RadDock's z-index being bigger than RadToolTip's z-index.
In order to achieve the desired behavior you must apply a CSS style which will override the default one as shown below:
| z-index:10000000 !important;
You need to use the keyword "!important" in order not to let the default style to override your custom one.
The ID which is used is the ID of the RadToolTip's wrapper element. If you want to apply the same style to a RadToolTip which is not shown using the RadToolTipManager, the appropriate ID will be:
All the best,
the Telerik team