Hi and welcome to the forum.  The system you are linking is nothing more than a battery maintainer (it will not charge even a single battery from any state of actual depletion).  FWIW I use 30watts on a boat as a battery maintainer.
The size of your solar system directly depends on your planned usage of the system and storage capacity (number and size of batteries) so can't give you a real solar size need estimate without a whole lot of additional information
Generically to have a useful solar system for RV minimal self sufficiency you are looking at 100watts solar and twin 6volt batteries as a minimum which might get you to being able to do a weekend without electrical hookups.  Personally I have 265watts solar and twin 6volts.  This allows me to do 3 day football tailgating without hookups.  If I were looking to be truly self sufficient I would start with 2x that (batteries and solar size) and add as necessary.