I did exactly that on my last travel trailer. The bedroom closet went across the entire trailer and there was storage below the closet floor. But you had to lift up the floor, which meant you could not keep much on the floor. So I added two out side hatches so I could access that area from outside.
Once you decide where you want the hatch, carefully cut away the inside wall covering from where you want the door. Then inspect this area for any wires or structural metal. If you must cut through a piece of structure, then you can add a horizontal piece of angle metal between two adjacent vertical structures.
Good luck.