It's too big an if, especially if you have a Mensan's grasp of probability. Sure, it's possible to do it without leaving any evidence (physical traces or witnesses). It's just too damned risky. See Lanza's post number 421 for a nice analysis of all the things that could have gone wrong. The prosecution's case would be a lot more convincing if they were to present an account of the abduction.
If I know that both the girl and Westerfield are at point A (the RV) and that the girl was at point B( the home). I can assume that the girl either went to the RV or that someone brought her there. Whatever of these things are true, ther evidence indicates she was in the RV after abduction and before she died.