Hi,
Did you check the class (CL_EX_BADI_SD_SALES) which has the method IF_EX_BADI_SD_SALES~SAVE_DOCUMENT_PREPARE which has access to all the tables.. and this is called just above your userexit in the program SAPMV45A. Otherwise you should be able to access the following internal tables in the userexit.
vbak,xvbap[],xkomv[],xvbapf[], and many others.. check line no 589 in the code MV45AF0B_BELEG_SICHERN.
Hope this helps
Regards