If you mean storage space below the floor of the trailer, there are few models of TT that have been built with basements, and I know of none currently built that are small. Carriage, for example, once built TTs with basements, but they were usuall pretty large, and the company was making only fifth wheel trailers for about the last ten years before the brand was bought by Forest River.
TT manufacturers will do "pass through" as storage that can be reached inside and from an outside door, usualy under seating or a bed. The same term in a basement RV means storage under the foor that goes all the way across the RV, you can push somthing in one side and out the other.
TTs might be said to have basements when the manufacturer encloses the utility space under the floor, where plumbing runs and some tanks maybe located. This is seldom deeper than the height of the frame rails at the frame (maybe 4-6 inches) and often tapers to 2 inches or less where the covering material reaches the base of the trailer walls.