If you want that stuff out, you'll need reverse osmosis. I'm not sure who told you that it was difficult and costly to install on an RV (unless you're trying to get *all* the water to be reverse osmosis).
I assume you're talking about drinking water and installing it in the kitchen is as easy as a residential install, probably a little more so as you've got direct access to the PEX.
The only question in my mind - if you need it in the bathroom, that's a separate install OR it means running a line from the kitchen install to the bathroom.
Remember, RO uses about 4 gallons of water to make 1. So it's not super efficient - probably not for off-grid camping.
You can easily buy a basic RO system and the various bits/pieces to have it installed for about $150.