There are no graves for the Jews who were allegedly killed at Treblinka, but there is a symbolic cemetery that has been built over the ground where the bodies were allegedly buried. No digging is allowed, so the bodies have never been found.
I blogged about Samuel Willenberg in this previous blog post: https://furtherglory.wordpress.com/2014/07/10/samuel-willenberg-91-the-last-living-survivor-of-the-treblinka-extermination-camp/
I also blogged about his daughter who has designed an Education Center for tourists at the Treblinka Memorial Site on the grounds of the former Treblinka death camp. https://furtherglory.wordpress.com/2013/08/14/90-year-old-survivor-of-treblinka-death-camp-unveils-foundation-stone-for-future-treblinka-education-center-designed-by-his-daughter/
The following quote is from a news article about Willenberg’s death:
Samuel Willenberg, the last surviving prisoner of Treblinka — who managed to escape the Nazi death camp in 1943 — has died at the age of 93.
In 1941, Willenberg’s two sisters were arrested in Czestochowa, while his parents used false documents to escape the Nazi purge. At the age of 19, he was rounded up with the Jews during the liquidation of the ghetto in Opatow in southern Poland, and sent to Treblinka.
Acting on the advice of another Jewish prisoner, Willenberg posed as a bricklayer upon his arrival at the extermination camp. He was the only person from his transport not to perish in the gas chambers.
Willenberg took part in the 1943 revolt at Treblinka, becoming one of the few hundred who managed to escape the camp.
His daughter, Orit Willenberg-Giladi, was in 2013 named as the architect to design a Holocaust education center on the site of the Nazi death camp.