Starting in 1997, the 208 Adventure came with a poly 82 gallon tank. At the same time, they stopped providing the large gas tank hatch which requires you to cut the deck in order to replace the tank. :hmm Maybe the thinking is a poly tank should last forvever? IDK. This was almost a show stopper for me when I bought my 1997 208. I wish there was a hatch in case the tank ever did need to be replaced. There is a small hatch (about 3x2) where the the water tank is located, maybe this is what you think is the smaller gas tank? I dont know why you are only getting 40 gallons though.
Here is a picture of the deck. The only way to see the tank is through the 2 round deck plates.