I know that this may sound trivial to you young healthy people, but I'm a big guy with bad knees, and I have a terrible time climbing up into the cab of my M35A3, so I came up with a couple of simple additions to my truck.
First, I bought a cable step (2 pieces of 1" steel cable mounted to a top plate and a bottom step plate) and mounted it to the under side of the running board. Now, I can get up onto the running board.
But now, there's that terrible lip on the cab to get my feet over. So, there is a jerry can holder on the running board. I took an old gas can that had rusted, and I cut it in half horizontally. I painted it with some heavy paint, made a new tie down strap, and mounted it in the jerry can holder. The can is now half-height. Nobody ever notices it.
Now, I first step on the cable step, then the running board, then onto my short jerry can, and presto! I'm in! It was easier than fixing my knees.