I agree with mb36912. Our RV came with an undersink carbon filter for drinking water. We added a two canister set from https://www.rvwaterfilterstore.com with a 5 micron and 1 micron filter.  In soft water areas, like much of the Pacific Northwest, this is sufficient.  In hard water areas, like Arizona and Florida, we put in a water softener and add a Brita pitcher to take more  of the salt out.  We have had the hard water mess up our toilet, faucets and hot water heater with deposits.  

Pre-filtering the water prior to entering the RV can be a good idea. It can have various benefits, such as: making sure nothing enters the RV's plumbing system that might cause an issue, that filter may be easier to regularly replace, and/or if the external filter is a lower cost filter, that can help extend the life of a pricier interior installed filter(s).

A lot likely depends upon what your existing filter system consist of and your desired goals. A prior conversation that may be of interest: https://rv-dreams.activeboard.com/t62399249/do-we-need-water-filtration/ , which includes different approaches to water filtration.