Well, not that easy to tell if it is nickel plated inside but not from outside, considering normal reaction of the acid on the testing spot. I wouldn't bet on that, but I could try to strip it clean with something stronger, let's say 15% HCl. I usually use it during nickel plating, even though not in such large scale, the tank in question has quite a surface area.
I've been also warned somewhere, because it is a gas tank, that longtime exposure to petrol products can affect the reaction too, even though I'm not sure how true is that. The tank was empty for years, recently thoroughly rinsed by water and acetone, followed by multiple phosphoric acid tries I mentioned before.
edit: I forgot to ask, IF it would be nickel plating, what is the surface now and how stable could it be?