Imperial units and line width

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Welcome!

Check Optons/Application preferences and Opions/Current Drawing Prefences. Both should have the same units, I have mm, you propably inches.

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Line widths are always defined in mm, even when the drawing is set up in inches. It doesn't matter if the drawing is in metric or imperial.
You can not define a different linewidth just for printing. You have to draw with the desired line width.
It can be defined directly in the pen properties, or via Layer attributes if "By Layer" is set.
Pen properties can be defined when drawing, or altered afterwards with Tools - Modify - Properties for a single entity or -Attributes for several entities. Also Layer Attributes can be changed afterwards.

You may upload a sample file here if you can't solve it.

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You might have draft mode activated.

But this only has effect in drawing view, not in Print Preview and real printing. Check Print Preview.

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I have opened that file.
If you zoom in very closely at one of the line ends you will see there is a thickness. I have measured this with the Measure tool and got 0.083. As this is in inches (your units) multiply it with 25.4 and you get 2.108, nearly exactly the set line width of 2.1mm.
The line width also reacts to a change. So everything is ok. The line only looks very thin in relation to the length.

The sample line is 90 inches long. Your paper format in current drawing preferences is Custom with 0.3255 x  
 0.46035 inches. ???
You said most of your drawings fit on Letter size and you prefer scale 1:1.
So the issue comes from your drawing setup. If you draw a shape that fits into letter format and set this properly, and in Print Preview set the scale to 1:1, everything should be allright.

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It shouldn't make any difference using Layer 0 or an other one.

What I have missed to mention is you might have had "Apply Print Scale to line width" on in Print Preview.
The purpose of this feature is to scale line widths  down if you want to print out let's say a drawing intended and set up as format A2 in a handy A4 size. Line widths would appear too thick otherwise. For normal use leave this button off.