I'd like to suggest you reset your expectations a little.
You have no "leverage" as a Five Star.
If you rent at SFO, you will have the relative convenience of picking up your car after your flight; you will - almost certainly - have a wider selection of vehicles to choose from - depending on the day and time of day you arrive. But you will also pay airport fees etc
If you rent at any of the other locations, you will save on some of the fees - hence the cost difference. But you will have a smaller selection of vehicles to choose from.
What's more important to you - 20% more money in your pocket or a marginally different car?
As you are renting for 12 weeks, you could do a number of shorter rentals (3x 4 weeks etc) and change cars ... but that means you have to return the car every so often and play the lottery of what may be available.