If you don't have the room in that compartment and you still want a water softener, that can be stored separately and put in-line between the campground hose bib and the coach.  This is a better setup as the water softener is after the filters but I have not found sediment to be an issue with my water softener.  I do back flush it about once a year but otherwise, I don't do anything special.  I have had mine for a number of years and someone mentioned a product called ResCare which I bought and used 2 recharges ago.  I think it made a noticeable difference in the performance of the softener.