Just taking a stab in the dark here, as I don't have a M2, but if you can fill on both sides of the truck, it's 2 tanks. There may be a hose in between them, so that they drain / are used equally when traveling, but I don't think you can get away with filling one, and it filling the other on the other side as quickly as the fuel will go in. At least that's how I understand semi's and dual fuel tanks.
Greg, Janelle, Tenley, & Ellis
2019 Palomino Puma QBSS
TV: 2015 Silverado Z71
Other toys: 2007 Rinker Captiva w/Monster tower & rack