It depends on the what the grain of hardness of the water being softened how much sodium will be in the water.
Hardness ranges from 6 to 25 grains per harness per gallon.
Using an average figure of 15 grain according to the WQA = 30mg in a glass of water.
The Cherrios you have for breakfast has 200mg in just one cup! :W
>>A quote from the Mayo Clinic (the doctor is referring to water hardness of approximately 7 grains)
“An 8-ounce (237-milliliter) glass of softened water generally contains less than 12.5 milligrams of sodium, which is well within the Food and Drug Administration's definition of "very low sodium." Says Sheldon G. Sheps, M.D. at the Mayo Health Clinic.<<
http://www.merleswater.com/blog/bid/144148/Drinking-Softened-Water-How-Much-Sodium-in-Softened-WaterI don't use one. I use the inline disposable blue filter which does quite well removing rusty iron water. Uses carbon for filter. IMHO No need for an installed water softener system.
If you are in a 'year round permanent site' with bad water then it would make sense to install one because you would have to replace the blue inline more often when parked.