Hard water has a lot of calcium and magnesium carbonate. When this stuff comes out of the water, it makes a hard deposit (like a rock). This happens anywhere the water is heated, or where water sits and then dries up. It's worse in your water heater, because heating causes it to deposit faster. You will also notice deposits that cause annoying problems around your shower head holes, the sink, at the water level in the toilet bowl, and a lot of soap scum in the shower.
I live in a hard water area, and the deposits don't normally cause problems directly in piping---it's at the point where the water comes out and dries up. I don't think it will cause you problems in the RV tank or plumbing, and the deposits are NOT harmful to people. They are just unsightly and annoying.
Best thing is to flush the water heater often and be prepared to replace the heating element often. You can remove the deposits on the shower head by soaking it in vinegar. Spray down the shower/tub with vinegar. Soak a paper towel in vinegar and lay it on the deposits around the sink & faucet, so the vinegar stays in place and works.
I doubt you have a big problem in the fresh water tank. But if you want to get deposits out of the plumbing, buy some cheap generic vinegar, and pump it in through the winterizing valve, without bypassing the water heater. Do not dilute the vinegar. Open up all the fixtures and run some through all the hot and cold lines. Be sure to leave some fixtures open after the pump is turned off. Dissolving the mineral deposits produces carbon dioxide gas, and it needs somewhere to vent.
Vinegar will NOT sanitize your system. So follow with a fresh water flush, then a bleach fill & flush.
Don't leave the vinegar in the system to mix with bleach. It is not harmful, but will break down the bleach so it's not effective.
