I don't know what your plans are to access the bed's deck.
Your bed deck may be low enough to jump up on, but
On the 1 ton & above trucks , where i once worked, the trucks had a either a 2 or 3 step ladder.
The ladder rails were just the right size to fit between cross beams under the bed.
To operate it, we would lift the ladders end about 1 inch, ( that kept the ladder in place while driving ).
then we pulled the ladder out & lowered it, so we could climb up or down.