I have created a sample project which resembles the desired scenario - a RadSpreadsheet inside a ContentTemplate.
In order to have the RadSpreadsheet shown properly inside the ContentTemplate the RadSpreadsheet should be refreshed when the RadWindow is shown and resized. The event OnClientResizeEnd is used to always refresh the RadSpreadsheet when a resize is done. The OnClientShow event is used to repaint the RadSpreadsheet when the RadWindow is shown.