If there is a topographic surface in the model, be sure it is turned on in that view and then use Properties > Visibility/Graphics Overrides > Topography > Cut > Lines to assign it the thickness you want. See attached.
If there is not a surface, drafting with Annotate > Detail Line seems the best bet, although you will have to make sure that all such detail lines in all such views correspond to the grade, which is not making the best use of Revit.
