Use the below query to monitor the currently running rman backup status.

SELECT SID, SERIAL#, CONTEXT, SOFAR, TOTALWORK,
 ROUND(SOFAR/TOTALWORK*100,2) "%_COMPLETE"
FROM V$SESSION_LONGOPS
WHERE OPNAME LIKE 'RMAN%'
 AND OPNAME NOT LIKE '%aggregate%'
 AND TOTALWORK != 0
 AND SOFAR <> TOTALWORK;