I use pool shock, about a table spoon in the tank a few days before trip ( from stored with dry tank) with about 10 gallons water added.
the granules are easy to handle.
let it slosh around while i go to costco and air up the tires, get supplies etc.
the chlorine vapour released will help to sanitize areas that the water mix doesnt hit.
run through all the taps and toilets- much of it going to the grey and black tank to clean them
allow to sit a couple of days
then drain fresh tank and add however much water you want (we don;t travel with full tanks unless headed for destination known to not have water- so usually we will have a just a few gallons.)
we have a pool, so used to dealing with chemicals and by smell how much is too much.
household bleach is usually 4~7%
liquid bleach from poolstore is usually 15~20%
sodium dichlor- the pool shock is a stronger concentration by volume- so just a table spoon will do it.
For the most part we use the tank water for flushing and cooking, cleaning etc, we usually carry 3 gallon jugs of known good water for drinking and coffee
I learned this here:
How to Sanitize Your RV Fresh Water System