I bought the Stromberg Carlson LA460 Universal Outdoor Ladder for RV last year and my kids have no problems getting to the bunk. they are 5 and 7.
I cut just about an inch or so from the bottom and put back the rubber feets just to get the right height and angle and the ladder was set beside the bathroom wall so that it wasn't in the way when you get in the RV. + it also serve as a tie down combined with a floor bike stand (keeps the bikes from hitting the inside wall from the bunks)