Upgrade coupons aren't really the thing they used to be, especially as more of them start requiring rack rates. Autoslash is your best bet and book or set an alert for the class you want. Your expectations for prices may need to be adjusted also. Gone are the days where you get a car from a major for $100 or even $200 for a week, best price I've seen from a publicly available code is $250. If part of your rental is Memorial Day, that is not going to have the same deals available you would have the first week.