Use the following workaround: create a second sheet with only grand totals, and then combine the sheets on a dashboard.
- Open the workbook in Tableau Desktop and select Analysis > Create Calculated Field.
- In the Calculated Field dialog box that opens, enter a name for the field.
- In the formula field, create a calculated field similar to the following, and then click OK:
where <Field> is the field or table calculation used in the view.
- Right-click the sheet for which you want to show grand totals, and then select Duplicate.
- On the new sheet, replace the original calculated field with the newly created calculated field, and then hide all but the first row of data.
- Right-click on the calculated field name on Rows or Columns and clear Show Header.
- Combine the crosstab sheet and the total sheet in a dashboard and format as desired.