I've never heard of them being used in RV fresh water tanks, but i used to use them in my swimming pool.
Maybe you can just drop one into the fresh water fill, and fill with water. Seems like that is about your only option. Are they stabilized like pool pucks, or pure chlorine like pool shock? The former is milder than the latter.
To prep our fresh tank, I add a small amount of bleach to a full tank of water, and then drive around to shake it up in the system, then drain it and repeat, and then just fill it with tap water.
Then it's hit the road time!!
btw, we don't drink fresh water from the tank, just bottled, so it's probably overkill, what I do.