If it does turn out to be the tank ,I had good luck with a "Universal Plastic Tank Repair Kit" by SYON, P/N 90214, SR-214. I think that a couple of companys market this kit, as the Brand names were different, but numbers were the same. $ 12- 13.00 at local RV store. Says you can patch in place(if you can get to the leak). Just don't do what I did
,stood tank on end, filled with water, figured if it would hold with that weight of water, I would be good to go, went in house , wind came up ,heard a loud "Woomph" , took 2 more kits to seal the seam.
Good luck, Toby