Sanitize or replace old camper fresh water holding tank?

We don't drink the water from the FW tank we just use it to shower and clean dishes etc. So I never sanitize it. Frankly bleach is more caustic and harmful to you and the environment than anything that would be in there from the usual filling and draining. More and more hospitals are using Hydrogen peroxide to sanitize than bleach for that reason. You can ingest hydrogen peroxide and breathe the fumes and it won't kill you like bleach can. Bleach is nasty stuff and it wreaks havoc on septic systems.