The real reason I fill the on board Well the ORIGINAL reason was super chlornation of the city lines. >Took forever for the taste of bleach to go away but I recalled from my Fish Tank days if you let it sit in the supply tank (NOT under pressure) that clears quickly (like minutes).. Sure enough it worked.
Plus the city water being chlorinated keeps the system healthy.
Now... I move from park to park every two weeks. My tank lasts 3-4 weeks. So Both fresh and waste (normally) so now I only hook up at one park At the other park I always get my favorite site cause it's 50 amp, Level. Water and Electric, NO SEWER so it's perfect for me.