We can make a variety of sizes for Portable RV Water Softeners (they're portable softeners and don't necessarily need to be just for RV's).  They contain a screw off top with a basket to replace salt and don't come with a valve that requires electricity, instead they have an in-and-out head for easy installation and adapters for easy hose connections.

These RV Water Softeners are portable, compact, travel-ready. This is a handy and convenient portable water softener system for anyone on-the-go with limited space. This water softener comes with garden hose connections for easy installation in your RV.