Magnetic hooks can also be used on the outside of the camper.
Besides towels, you can also hang up an awning or light chains, for instance.
Protect the car finish from scratches
It is important to protect the car paint when a strong magnet has direct contact with it because it may leave scratches when you remove it.
This is why rubber-coated magnets are ideal, because you don't have to worry about scratches.
In the picture you can see the hook magnet FTNT-40-B
As an alternative you can use rubber caps
If you don't have something like this at hand, you can use a microfiber cloth as a protective underlay.