Even with dually wheels, it's usually referred to as 2x4 (or 4x4, if the front wheels are driven), as the "by" specs are for axle ends, not physical wheels. I know, things are not consistent.
The weight generally does help keep some traction on a class C, at least compared to some other vehicle types. It also, I would think, makes things rather more difficult if everything breaks loose at once. I have no desire to find out how my motorhome behaves on ice or similar surfaces.
Some years and models of the E series chassis have a drive shaft mounted parking/emergency brake, bolted on the back of the transmission, rather than ones actuating the wheel brakes as is more commonly done. For those years, it's obviously fruitless to try the trick of applying the parking brake lightly to force both wheels to turn. (It's also absolutely essential to securely chock the diagonally opposite wheel before jacking up one side of the rear axle, of course; once one wheel loses traction by being lifted off the ground, the parking brake has no holding power at all.)
