Let's say c100.dwg is your sheet file ( if you don't like layout tabs). Your Site.dwg file is your base file. You have a survey file Survey.dwg, a Building file from an architect Bldg.dwg, and drawing X.dwg a drawing showing truck turning radii.
Your Site.dwg file will have the Survey.dwg and...
I ran into the same problem and here is what I have found out that works.
LWDEFAULT = Default lineweight ex .25mm = 25
DIMLWD = -2(default)
DIMLWE = -2(default)
Let me know if this works for you. It takes a little bit of playing with, but once it's set I haven't had any problems.