Some RV manufacturers install filters, some don't. Even with a factory-installed filer, some RVers use an additional in-line filter, and some install an under-counter filter at the kitchen sink for drinking water only. We use a Pur filter pitcher because we like to keep drinking water cold in the fridge.
Some filter systems have 2 or 3 filter housings, some have only one. Most of the housings will accept standard 10" filter cartridges, which are available for a variety of purposes such as removing sediment, taste and odor, harmful chemicals, cysts, etc. If you have multiple filters, you may have a coarse sediment filter in the first and a taste and odor filter in the second. We only have one filter, so we use a combination cartridge like this one
from Hydro Life.
There are portable water softeners like these
that are intended for RV use. They can be used in combination with water filters. If you use a portable water softener, you may want to add an additional sediment filter ahead of it.
Water softeners exchange sodium for calcium and magnesium in the water, which are the chemicals that make the water hard. The sodium is in solution and should not bother a filter located downstream.