Welcome to TBN!
I guess you are talking about the big plastic boss at the bottom of the tank. That is where the fuel valve goes (I don't see it), but it is the low point of the tank. Never tried to take the large plug out of the tank though. Outlet valve is mounted on a plate that is bolted to that.
Last year of the D-Series skiploaders. 8x8 with PTO. Emissionized engine. Grease the rear pivot of the front axle as dirt can get in there, cause oil to leak out and ruin the reversing box gears (box that it part of the pivot on the front axle). Expensive to fix.