The RadDockZone has a default padding of 4 pixels, which could be easily removed with a bit of CSS, but I am not sure whether this will help in your case. From the screenshot I see that there is a space between the dock's right side and the zone, that looks greater than 4px, and it might come from the table the zones are placed in.
Please try the following CSS on your page and in case the issue persists please send a sample runnable project, so we can debug the problem locally.
All the best,
the Telerik team