Mountaineer Tank Size
While I've never ran it to truly empty, I have pumped 19.18gal into a supposed (according to the manual) 18gal tank. Which leads me to suspect that the tank is actually a 20gal tank.
You are on the right track, you can put more fuel into "tank" than manual states. The Merc Mountaineer DOES have an 18 gal tank, but you add in the fuel filler line (the tube that runs from the tank to the filler nozzle where you fill gas at) and you can get almost 20 gallons on an empty tank.