When people rent cars one-way, the car rental company needs to return the car. In countries such as Sweden, Norway, Australia, New Zealand, those cars are listed online and people can rent the car very cheaply or for free, provided that they move it from A to B within a specified time and don't make big detours. Is there any such service in the USA?
There are a few. Each will come with their own terms and conditions, so check carefully (eg do you have to pay for some or all of the fuel, what about insurance, etc):
and if you want an RV:
In the US these are typically known as driveaway services. Many allow for any licensed driver to participate, but some hire drivers and are not open for public participation.
A typical example of such a service requires that drivers deposit a small bond (security deposit) and, for foreign drivers, bring a copy of their driving record from their home country. The deposit is returned to you after you drop off the car at the destination.
A few examples of driveaway services I was able to find: