An old thread...

http://www.horsetrailerworld.com/forum/thread-view.asp?threadid=15674

Some trailers will have a "boot" box like structure that serves both as a sitting area and can contain a water tank or various LQ systems. Some are carpeted, some have cushions and are upholstered. They usually are ~18" high, a common chair seating height, but not high enough by themselves, to provide easy access to the bunk area. I've seen multiple level storage units (cabinets) constructed to act as steps, and even counter tops used as a second step.

Basically it takes the height of the knee wall, divided by three, and installing "steps" on the bottom two, to provide easy access to the bunk. Their tops would be 18" and 36" high from the floor.  Moveable ladders or boxes are possible, but don't provide the same security or storage capabilities as thought out appointments.

There are many ways to fabricate built-ins, to suffice for both storage and access to the bunk area. Spend some time in the trailer for sale section, and look at the many interior photos. You may find one to copy that will work for you.

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=375789

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=368019

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=375719

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=375571