If you bought an ABS tank, you will have an easier time installing it in your trailer. The drain fitting will be mounted on the bottom of the widest part of the wedge. The valve will be attached to the flange and plumbed with ells to where you want it to end. It will be terminated with a coupling and cap.

Be prepared to mount and remove the tank a few times until it is finally secured into place for use.

It is very difficult to predict exactly where the supply fittings will be located until the tank is in its place under the floor. Then you can arrange your indoor plumbing, and know exactly where it will terminate above the tank. Marking the floor, and later drilling a pilot hole through the floor into the tank will give you an exact reference point, for the stack vent, drain lines, and toilet flange if this is a black water tank.

Each installation is different. I would suggest taking a camera to a trailer sales or friend's house and photographing their tank's installation. It's easier to see the final results than try to describe how it's fabricated.

I was looking for tanks for my trailer.  which is a Adams. Gunslingers do there LQ's.

I called them and they sold me tanks and mounting hardware for less than any I could find on the web.

just a thought