The walls curve out in the middle, so things are kind of wonky to measure. The widest part inside is 91 inches between inside panels. The floor itself is 19 feet long, and 92 inches wide (but the walls are two inches thick, so the exposed part of the floor is smaller). Ceiling height is 79 inches from the floor, at the center, but it curves down towards the sides.
Keep in mind that there are several intrusions and holes that you might want to work around, such as the battery compartment, water heater opening, furnace vent, refrigerator vents, air and sewer vents in the roof, windows, and the door location. Along with the wheel housings.
In other words, don't get too far along with your planning, until you carefully measure your actual trailer!
I measured a 1972 Land Yacht Safari, two axle trailer, with the front and side beds, and rear bath. This trailer is called a 23 foot, and may or may not be the same as the one you are getting.