We use MS NAV too and I have made some QV apps from it. In my experience you will need to speak to your NAV provider to obtain the table descriptions you need because they are not especially straight-forward - although they are very well structured. Also, the field names often need an explanation.
This was my approach, but I have never used the sales tables.
Let us all know how you get on because there must be many NAV users out there !
The customization will make it more compliacted. However, the basic structure of the data should remain the same in an area such as sales. But it is even more reason to speak to your provider. Their developers must have a very good understanding of the data.And of course, once you get the connection from QlikView it is fairly easy to use 'trial and error' to explore the tables.
Were you able to resolve the issue or get what you're looking for?
If not, I just thought I should also included what I knew with Nav tables to work with QV.
Basically when we talk about sales in Nav we consider the following tables:
Sales Invoice Header
Sales Invoice Line
Sales Cr_Memo Header
Sales Cr_Memo Line