I feel like the rental car may have been acquired because G was perhaps deceased for a time in one of the other vehicles and she didn't have time to clean it yet (likely the truck), or there was evidence in it, and she didn't want AS to be suspicious of anything. Even though the rental car itself is suspicious.
This reminds me of when Josh Powell had his minivan but rented a car and drove like 400 miles in it likely disposing of evidence. I feel that is perhaps what TS did. Not disposing of G himself, but any evidence, etc, so that it can't be tied back to her (she hopes, anyway).
MOO obviously.