Epistle of Rabbi Jochanan Ben Zakkai an Obvious Forgery? And Was He of Davidic Descent?

The "Epistle of Rabbi Jochanan Ben Zakkai" which is (allegedly) based on the last will and testament of his teacher Rabban Gamaliel is an obvious forgery.

The epistle was alleged to have been by Ben Zakkai to the Jews of Rome warning them about the heretical new sect called Christians. It mentions Peter who is called "wine son of vinegar", a reference to his having been a disciple of R' Gamaliel (I thought that was supposed to be Paul....).

Interesting reading, nonetheless.

http://www.hebrewbooks.org/37982

Paranthetically, where does the OU get this?

He was a descendant of the House of David and his family has kept alive through the ages the hope of the People of Israel for the arrival of their Anointed King, the Mashiach, may he arrive soon, and in our days.

 A colleague of mine is convinced that the 'sons of zakkai' who returned from the Babylonian exile were from the royal house. The name also appears often in the Exilarchic line. 

http://biblehub.com/ezra/2-9.htm

To further confound everyone, according to this he was from the Tribe of Levi.

What tribe did youchanan ben Zakkai, come from?

Levi.