I rented a car from National almost straight for 2 years. At my location, the taxes and fees are insane, but I found that they are also capped. As such, doing weekly rentals, even with the BJs coupon, was more expensive than renting for 2+ weeks at a time.

The airport and concession fees would cap out after 2 weeks, meaning that the marginal cost of each additional day was significantly less expensive. It may be worth doing the math on your rental. If you have the benefit of having a non-airport National location, you can also avoid a lot of these taxes/fees, although the car selection often leaves something to be desired.

Finally, don't hesitate to negotiate with the station manager for a rate (the real station manager, not the "manager on duty") if you're going to be a long-term renter. They know the economics of their station, and you can probably get a significant discount from the published rates.

In terms of rental credits, I believe if you're doing back-to-back rentals from the same location it is considered one continuous rental from a credit-accrual perspective.