Just saying here ... in about any RV water tank I'm familiar with (60 gal) ... a single cup of clorox would be a plenty ... otherwise ... it'll take forever to get the taste of clorox out of the system.
I think you are right about the quantity of bleach to use. I would have been better off to have used a small bottle of bleach rather than the gallon. Oh well. I guess I have a lot of flushing of the fresh water tank to do