I was looking into ordering a similar size tank like you are looking for to add under my jackknife sofa. I wanted more water capacity for boondocking to supplementing my 36 gallon tank mounted under the trailer. Instead I purchased a 50 gallon tank for the truck bed. Grey water disposal was not an issue for me camping on private land.
Plastic-mart has a lot to choose from. They have 5 different 42 gal rectangular tanks but to fit the limits of your dimensions, you may have to go with a 40 gallon tank. At the left side of the column when you click the tank size it will take you to another page. There it will show you the standard layout for fittings they will add to one end of the tank. One is a 1-1/4 npt female at the top Center, 1/2 inch npt female at the top left corner and 1/2 inch npt female at the bottom center. The standard fitting layout is $20 extra. They do custom fittings where you want them on the tank. You will have to print off the order form and mark the drawing showing the location and sizes of the fittings where you want them located, but it will cost more.