Then i undestand ( i think ), you are using standard wall (1 component ) that follow the same line when making wall/ slab
As Nikki said, if you use 3 componen wall the line will show in plan view, but there you can use a solid brush in black/ grey/... and do a draft render.
Next option is to make a polyline that follow the outerside of the building, offset that in i e. 1/2 inch and use the "Convert to slab" tool on the polyline and you cant see the slab in elevation, next wall stand on the top of last story, but then you will se the line for each story, to avoid that you can make the exterior wall go all the way up to the top so you only have 1 wall heigth.
Then in a section view i would explode the section and trim the line from the inside of the exterior wall thet is going tru the slab's
Change the printersetting, draw a white line is to mutch work for me
As Alvin said, in real world the wall will be min. 2 or 3 component.
I use the baseline to set the wall/ slab after.
Make a closed polyline that follow the foundation, copy it up in different Z's for each level, convert to slab and the use the line for convert to a component wall that follow baseline.
Just remember that after convert to wall the line go away, so it's best to make the slab first.
V18, V19, V20, V21, 2015/ 16/ 17/ 18 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB