In the novel To Kill a Mockingbird, is the "knothole tree" the same tree that Jem and Scout struggle with Bob Ewell under?

Yes. In Chapter 4, Scout mentions that, as she ran past the Radley's home, she noticed something shiny coming from one of the two live oak trees at the end of the Radley yard. She specifically mentions that the roots of the trees were large and made the side-road bumpy. Scout then approaches one of the trees and finds two pieces of chewing gum in the knothole. Later in the novel, Boo Radley continues to give the children gifts through the same knothole in the tree until his brother fills it with cement. In Chapter 28, as the children are walking home from Maycomb's Halloween festival, Jem tells Scout they are approaching the big oak. Scout then says she feels the sand go cold and could also feel the bumpy roots of the tree. After Bob Ewell attacks them, Boo Radley comes to their aid by fighting him. In Chapter 29, Scout recounts her story to Sheriff Tate and tells him she knew they were under the big oak tree because the ground was cold. Scout also mentions that she could hear someone being thrown against the trunk of the tree. Since there are two big oak trees in the Radley yard which stand side by side, one can surmise that Boo Radley struggled with Bob Ewell underneath the same tree where he placed gifts in the knothole.