Terry's right...prob'ly not the tank itself
Unless the tank is leaking, it's rarely if ever the source of odor INSIDE boat...'cuz unless it's leaking, the only place any odor from inside it has to go is out the tank vent. The most likely sources of odor inside the boat are permeated sanitation hoses, a wet dirty bilge, a dirty sump. Hunters in particular are notorious for areas where water can become trapped, stagnate, and make the boat smell like a sewer. A PITA to deal with, but curable.None of the above are reason to reject a boat...but they make do make excellent bargaining chips. The only thing that's likely to cost you--better yet, the seller--any money is new sanitation hose. So if you like the boat otherwise, make an offer "subject to survey"...hire the best surveyor you can find (one who's highly recommended by those who've used him/her to BUY a boat, but not necessarily liked by brokers and sellers). If the survey reveals that odor is the only major problem, that'll take some serious elbow grease to get rid of it, but it can be done.