Hi,
Why don’t you go for Function module based generic datasource, being you are required to fetch data from multiple tables?
Always you have be conscious while getting data from condition tables (You have mentioned that you want data from KNOV table). Always you get multiple line items for each Billing document.
Better, try to use 2LIS_11_VAITM, 2LIS_13_VDITM and 2LIS_13_VDCON.
- Vijay