Offset surface to object. (200'/*300' with 6" thickness)

Hi all,
I have been trying to offset my object so it has a thickness so I can continue modeling but I still couldn’t find any solution for it now (I also tried Maya, but I’m not really familiar with it. Edit Mesh_extrude_, but it still not working).
The model scale is about 200’*300’ with a thickness of 6".

These are some of the photos that I failed offsetting.
First picture is the whole model, second one is what it turned out.

Even I cropped out a little piece to try if it would work. (the first picture is the piece, the second is how it turned out)

Here is the file:
Model.3dm (11.9 MB)

Thank you!

It is difficult to help without the .3dm file.

Sorry about that, I have attached the file.

Thank you!