The strategy would be the same if you wish to keep it that way.
Your main application can present the main menu and then you can have separate Class Library projects for Customers, Proposals, Sales, etc.
Each of these could have their own set of winforms for presentation to the user but it is normally best to separate the UI portion of the application from the business logic so you may wish to handle all of your presentation logic within the main application and then just call into these other modules as required.
I will try to put together a simple demo and then attach it to this post.
If others out there could share their own experiences with packaging of a WinForm application, that would be great.
Thanks.