Which Bitcoin RPC call is used for setting transaction priority?
The transaction priority is implied by the fee which you set for it. You can estimate the fee with your desired priority using https://bitcoincore.org/en/doc/0.16.0/rpc/util/estimatesmartfee/ 

Simply compute your transaction size (bytes) and adjust the transaction fee according to the sat/kB returned by the estimatesmartfee call.