Enterprise is the one most likely to pick you up and drop you off as that is part of their philosophy. But it is an office by office, staffing by staffing decision. During peak rental times and periods, they just may not be able to accommodate you on your schedule so call and speak to them in advance.
If your flexible and willing to work with them, you'll be surprised what you can get. I reserved for a busy Friday and returning Monday. They offered to pick me up late on Thursday (I mean 9pm), but only charge me for weekend special if I agreed. And I got a full size luxury car at midsize car rates just because it was a special rental that was returned and it wasn't schedule for picked up back to the pool until Tuesday so to them it was banked money. Flexibility get the best results.
Also check local shuttles in touristy areas as they may stop near the campground and where the rental cars are located. And as said, check with the campground office as there could be some arrangements they can make for you.