In order to list a rental property you need to be registered as a landlord, property manager, or real estate agent on the Trulia site.
Currently, you are registered as "Other/Just Looking" so you would need to change your status. Then you can click on the "Rent" tab at the top of the screen and click on "Free List a property".
I'm not certain if you are able to list a RV space for rent but you could try it.
Prudential Connecticut Realty