In the select query it seems object level parallelism is enabled on RHS_PREMIUM_1 table. Not sure how much it is. It is always preferable to use parallel hint rather than object level degree. Please check and increase the parallel threads.
Also as you are doing conditional insert, please check if you can use partition column to get partition elimination advantage (for RHS_PREMIUM_1 table).
As the target table is truncate and load use 'append' hint along with parallel hint. Also, make sure target table is in nologging.
Try hard to get what you like OR you will be forced to like what you get.