Because you want stability and have some knee issues, I recommend something like the Torklift GlowStep Revolution Scissor Steps w/Landing Gear-4 Steps, Item # TLA78004.
The Torklift, Item # TLA78004, has aluminum steps that are lightweight, fold up for storage, and are 8 inches deep x 22 inches wide. You can fine tune the height and level of stairs with the built-in landing gear legs which also make these steps more stable. The mounting base dimensions are 27-7/16 inches x 12 inches... view full answer...