Originally Posted by Mr Havercamp
It won't work. The piping is ABS, the tank is MDPE. They use a gray sealant that does not dry in that joint and then clamp it. I believe its some type of a butyl product. I'd call Coachmen and ask what they use. The tank is most likely made by Alpha Systems but you'll get no help there. Their customer support does not exist.
5 picker has the right idea although I doubt you can get a Fernco fitting that will fit the outside diameter of the 3" coupler, and the outer dimension of the outlet of the tank, which is a bastard size.
As far as securing the tank, on the Roo's Rockwood jams a couple pieces of bead foam in between the tank and floor and boogers it all up with caulk. Its a highly technical way of doing things and very cheap and fast. I guess Coachmen does nothing? Maybe you could try the Rockwood method.
Yesthe materials are different but the abs cement will work and then clamped . it not really under pressure and it will seal with the use of abs cement and a clamp . if that's not the way one would want to go then just a good silicone will also do the trick , no sense turning a mole hill into a mountain this is not rocket science