Quick take
Use an inline filter for basic campground sediment/chlorine, a portable softener where hardness is severe, and countertop RO or a validated portable purifier when drinking-water treatment needs to go further.
Best basic hookup category: inline sediment + carbon
An inline filter is inexpensive, high-flow and easy to replace. It is best for sediment and taste/odor rather than broad dissolved-contaminant treatment.
Best hard-water category: portable softener
Full-time RVers in hard-water regions may benefit from portable ion-exchange softening to reduce scale in fixtures and the water heater.
Best drinking-water category: countertop RO
Countertop RO can provide higher-purity drinking water without permanently modifying RV plumbing.
Best travel backup: filtered bottle
Water-to-Go and similar purifier-style bottles provide a separate portable path when the RV is not connected to a trusted source.