Copy the following into a batch file:
::Not sure if this is needed
::It guarantees you have a canonical path (standard form)
for %%F in (%destination%) do set destination="%%~fF"
for /r %source% %%F in (.) do if "%%~fF" neq %destination% ROBOCOPY "%%F" %destination% *.rfa /COPYALL /R:0
Now, change the text in red to suit your source and destination (my destination folder was empty). Run the batch file and you should have a stack of RFAs all sitting in that folder. Note: results may be a bit unpredictable if you have duplicates of particular files in the source directory tree.
file - ROBOCOPY - Copy folders content to a single folder - Stack Overflow