If you really need the space, let me suggest you reserve a suitable alternative, maybe a 3-row crossover. It's possible Avis will not be able to move enough vehicles in to serve PDX demand; all big stations other than SEA are quite some distance away, and it, too, has quite a seasonal peak of travelers.
That's a good idea on the 3rd row crossover. Any suggestions on vehicles with a luggage rack on top? I am unfamiliar with these type of vehicles. One row will have my kids in car seats and the other will have my smother in law.
Avis' Standard Elite SUV will often be a Chevy Traverse or GMC Acadia, both offering 3-row seating and useable space behind the 3rd row. The Acadia has a standard roof rack; the Traverse does not. The Chevy Tahoe as Full Size SUV is a lot more trucky but also has a roof rack.