Not necessarily. I mean, they decided he was dead at some point, but that was not particularly early. Perhaps they impounded the second rental car and it was positive for cadaverine (so hence the knowledge that Gannon was dead - but only as related to that rental car).
Can you imagine if he was still alive when she gave the Cousin It interview? That someone really was helping her out? I mean, I don't think so - but this case is so bizarre, I'm reluctant to jump to any conclusions.
What could be her reason for traveling SO far with a dead body in (trunk of?) the car? Why not leave him in some deserted place in Texas, for example?
Maybe she was running entirely on panic, and without any phones. Who knows. Maybe she got lost on her way to SC - but even so, why keep the body so long? It does speak of a certain reluctance to admit to herself what she has actually done and to treat Gannon slightly better than trash - but in the end, she treats him as trash?
I'm sure we'll learn more as this goes along (such as how long he's been dead; presumably since Jan 27...but we just don't know).