Sanitize or replace old camper fresh water holding tank?

Before replacing anything, I'd do a very, very strong sanitizing first. Maybe even twice. Drain, flush very well after that till no more smell or chlorine or whatever sanatization process you use. Then fill the tank. Let it sit a couple days. Then get a sample of the water and find somewhere to have it tested, like a "Culligan" Water Softener company or some place like that, that can truly analyze your water (and maybe even do it for free). Depending up the results, either keep the tank as is, or replace it then.