I don't doubt her dad may have bought the van and paid the insurance. Maybe paid the property tax as well. Even without paying rent, earnings from jobs like Taco Bell only go so far. And we know GP and BL traveled to the west coast before the current trip. Both trips would have been expensive. Chances are they liked to do things like go out to eat even when not on cross-country trips.
And no, we don't know when GP got her FL license except it was before she was 21 since it was vertical. And to comply with the law, she'd have to have gotten it within 30 days of moving to Florida.
New Resident - Welcome to Florida!.
And you are right we don't know where she got her mail. But I'd think to get it in NY would have been pretty inconvenient. And dad didn't live in FL when she first got there.
None of us truly know what GP thought about much of anything. So we project our own thoughts and feelings onto her. But I know when I was 20-21 and didn't live at home, I would not have wanted all my mail to go to my parents' house. And honestly, I can't imagine living with someone yet not feeling it was enough of my home to get my mail there. But maybe that's what she thought. Sad.
JMO