Lets start with your request because you have made an interesting one, and wise beyound what you know. If you are asking for good fuel tanks then you are probably looking for used. Now here is what you need to know.
In the RV industry, many motorhomes and travel trailers/fifth wheels are built around the refrigerator. In other words, when the vehicle was being built it was easy to get the fridge in, but try to get it out when you go to replace it. Its not at all unusual to have to remove the door frame to buy an extra inch of space. And often then the door frame removal doesn't work.
Now with boats, particularly older trawlers, the boat was built around the fuel tanks and engine. I exaggerate a little but not a lot when I say they almost have to remove the back third of the upper structure of a boat to get at the engine and tanks. Often having to cut part of the deck away. So with this in mind, pay attention to accessibility of engine and tanks. And with older boats, replacing fuel tanks isn't an unusual occurrence. And it is difficult to tell how "good" the tanks are if 1) you can't see in 2) you can't see all around the tank, corrosive problems might occur where you can't see them. I just replaced fuel tanks on my 50 year old boat just based on the principle, even if something wasn't wrong, something would be wrong later. My refit guy recommended against replacing them but when he took them out and drained them all kinds of crude came out (gas tanks). I also replaced my water tank and same deal, crap came out.
Most things aren't a big deal if you have the money, but it can be a big deal if you don't.