On what basis does the RCC say they can claim traditions to be equal to scriptures and on same level for setting doctrines and practices?
CANNOT be from the Bible itself, so where?
It is very simple. Lets start at the begining.
The Apostles were commanded by Jesus that
Then the eleven disciples went to Galilee, to the mountain where Jesus had told them to go. 17 When they saw him, they worshiped him; but some doubted. 18 Then Jesus came to them and said, “All authority in heaven and on earth has been given to me. 19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.”
We can see this by several accounts
16 “Whoever listens to you listens to me; whoever rejects you rejects me; but whoever rejects me rejects him who sent me.”
This shows apostolic authority
and repentance for the forgiveness of sins will be preached in his name to all nations, beginning at Jerusalem
notice the tradition of reptentance for the forgiveness of sin is preached not writen.
Therefore we are sending Judas and Silas to confirm by word of mouth what we are writing
Notice here the writen word is confirmed orally by people given apostolic authority not on its own basis. Note Paul he says
But what does it say? “The word is near you; it is in your mouth and in your heart,”[a] that is, the message concerning faith that we proclaim:
faith comes by proclimation not what was written. Thus Peter gives us insight to how things are passed on
So I will always remind you of these things, even though you know them and are firmly established in the truth you now have... And I will make every effort to see that after my departure you will always be able to remember these things
Thus Peter's teachings were done orally and he insured that what he taught was passed on. Note how the first large group of converts were instructed
They devoted themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer
Note it wasn't devoted to reading text. Next here is an indicator of Apostolic tradition not specified in scripture
7 On the first day of the week we came together to break bread. Paul spoke to the people and, because he intended to leave the next day, kept on talking until midnight.
In other words the Apostles dedicated the day of worship to be on Sunday not the traditional sabbath day.
Now look to Oral tradition which Jesus adhered to
and he went and lived in a town called Nazareth. So was fulfilled what was said through the prophets, that he would be called a Nazarene.
This is not found in the Old Testement but is an oral prophetic tradition.
Then Jesus said to the crowds and to his disciples: 2 “The teachers of the law and the Pharisees sit in Moses’ seat. 3 So you must be careful to do everything they tell you.
Jesus wants people to adhere to teaching espoused from Moses' chair. This is also an oral Jewish tradition which is not found in the OT. Note where did Paul get this statement from?
In everything I did, I showed you that by this kind of hard work we must help the weak, remembering the words the Lord Jesus himself said: ‘It is more blessed to give than to receive.’
I don't think you'll find Jesus saying those words recorded in the gospels.
Or where did Paul get these words?
This is why it is said:
“Wake up, sleeper,
rise from the dead,
and Christ will shine on you.”
An early Christian hymn which is Tradition.
Thus we can see evidence of Tradition being relied on in the NT. And note all of this happened 20 years before the first gospel was written and since some of these things can't be found in those passages we must concure they were taught by the Apostles and held to be truth by Christians with out reliance on the written word. Thus we see liturgical worship, Sunday worship, Christian hymn being passed on by the apostles which aren't expressed in the gospels. Thus these things are important and scripture is just another part of Tradition which is actually writen. Tradition and Scripture often over lap.