Those are known as water softners - same thing used in your house but scaled down to use in your motorhome. RVWaterFilter Store is a good source.
These are not filters, btw. They use a an ion exchange resin to remove calcium. You regernerate the resin with plain old table salt. They are availalbe in variouse sizes depending on how much water you use.
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (sold)
2012 Tiffin Allegro 32CA (traded)
2012 Honda Fit