extended fuel tank for C class

I would find a reputable company that does this and ask them. They will have the experience to know if your particular rig is able to handle (and have the logistical space) to install a second tank. And if they aren't certain, a drive to their business can get a proper inspection to answer the question.

They can also give you the pros and cons of various installations. Separate intake ports, separate lines to the engine with a switch, both or neither.