He's on it! Serph doesn't react to Donnie climbing out the window onto the top of his truck; seems like a perfectly sound sequence of events to him, if Donnie's intent is to make a jump across to his vehicle.

Fortunately, Serph's vehicle is also quite tough. So while it doesn't handle the thorns as easily as the Turtle Tank, he's not completely reliant on it to clear a path for him. It's a bit bumpy as he pulls ahead though.