I actually found a trick to rental cars is check the prices at different locations. And I mean for the same company. It used to be that getting a rental from a major airport was generally cheaper than other locations. That isn't the case anymore, at least not in California. We have a small commuter airport nearby and the cars are generally cheaper there.
Also most rental companies like customers to use them exclusively. So they have membership type things you can join. I have one through Avis so I always rent from Avis and I get their discount. I picked Avis because they have more locations here in California and they are at most of the commuter airports that I used to fly into when I was flying myself around. They also seem to be cheaper when I compare prices.
But like somebody else said, instead of trying to get them to come to you, just stop by the rental location before you go to the campsite and pick up a car for the wife to drive and have her follow you to the campsite.