I think the most likely reason he drove it back to his parents is because he could afford to do that and he's not very bright. He was panicking and needed to be home, needed his parents at his side, so that's what he did.
It also allowed him to disassociate to a great degree from the crime. As if he was just on a solo road trip.
Did he have the funds to get a plane ticket? Wouldn't abandoning the van in WY or anyplace else have been equally suspicious? Notice that the way he did it, you are able to come up with a plausible scenario the exculpates him (she went off on her own). If the van had been abandoned, no matter where it was, it would have set off a nearby search effort when discovered.
How does it not look WAY more suspicious if, immediately after Gabby is last heard from, her BF abandons the van and flees?
Do you think there was something in the band he needed to dispose of? Like a body? If so, choosing to drive cross country (and then lawyering up) is a good way for a murderer to cover their tracks.
Anyone remember any other cross-country murder cases like this one??
(And I do believe there's foul play here; so if it turns out not to be a murder, it's still a really bizarre case).