The overhang may or may not have tanks. Just depends on the manufacturer where they place the tanks and the length of the camper as to whether or not the camper hangs over or not. The bigger the camper, the more "stuff" inside. Campers that fit entirely in the bed, in the case of a longbed truck, 8', can have full baths which most often are "wet" baths, that is the shower area and sink are shared with the loo. You're right about "basement".
You're talking about purchasing a pop-up which will have limitations but they can come with quite a few of the features that are also found in bigger hardsides.
Go to an RV dealer that carries Palomino, or Four Wheel for example. Leave your wallet home but let them take you through one and answer your questions.