[Fulltiming]

Bill, unless the 'black sand like material' is charcoal from the filter, that isn't good.

[whatsnewell]

I would think that a one time occurrence of sand, any color, in your water tank could be attributed to some sort of repair to a water line where you last filled up your tank.  This happened to us when we worked at the Indy 500 a few years ago.....they had to repair a break in the water main while we were there and the result was a lot of red sand in our tank, which we removed at a later date.  How we got it out is a whole other story.  A recurrence of the same material may mean something is going wrong somewhere in your plumbing system.  

On another note, we installed an RO (reverse osmosis) system in our coach about 6 years ago, and when we are using water from our fresh water tank, just as we do when on city water, the water goes through that RO system, plus there is an ultraviolet light that will kill anything missed by the RO filters.  I'm sure the fact that we live in our coach full time, and the water in our fresh water tank is continually being refreshed, plus the use of the RO keeps our fresh water tank pretty clean.  I have only added bleach a few times over the years.

We have a portable 'ZERO WATER' purifier we use when dry camping to purify the water coming out of the fresh water tank, and it comes with a testing pen that will tell you how pure the water is......whenever the water coming directly out of the tank gets to a certain number on that pen I add bleach, which I haven't had to do in several years.

That's my two cents and I'm sticking to it.....