RT.SSIt is not just the water hardness, the waste sediments and bacterial growth all plays a role in holding tank hard scale build-up, hard water just makes things worst. I have not seen any mineral scale build-up on our fresh water tank, and it we have hard water. I have thought of pouring vinegar myself, maybe if it's done regularly it would help.
Yes, you have to make a hole in your tank, I've been thinking of doing it for years just never had the courage to drill a hole in the tank...until recently. There are 2 types of permanent tank rinser in the US market, the first one is called "Tornado" it's is rotary action rinser and the no mechanical part spray sytem counterpart called "No-Fuss Flush".
Link for "Tornado rinser": http://www.campingworld.com/shopping/item/tornado-permanent-tank-rinser-with-remote-connection/28091
Link for "No-Fuss Flush":http://www.campingworld.com/shopping/item/no-fuss-flush/3495
I have a 2008 RS and I cannot see where I can mount the no fuss device part way up the side of my black tank. Mine is surrounded with shields, etc. does your SS have good access to the side?