How are the black and gray water tank vents and drains attached to the tank?

[Steveo57]

If you have to drop a tank down how do you deal with the drain and vent lines penetrating the top of the tank? Are they glued in, clamped on or what ???

My tanks are tight against the floor under the trailer and there's no access to the top of the tank where the drain and vents go in without tearing open a wall.