Most portable water softeners are designed to be placed on the ground between the water spigot and the coach. If your filters are installed in the coach then you are going to have no choice. If you have some flexibility in where this unit will be installed, I would install a sediment filter, the water softener and then a charcoal filter or a secondary finer sediment filter followed by a charcoal filter.
In my own coach, I have the spigot, the water softener and then a sediment and charcoal filter internal to the coach. It has worked very well for many years.