Have you googled it? The only one that comes immedialty to mind is Safe Harbours, bet there are lots

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>

Yes, I googled a number of combinations and generally come up with manlifts or stairs or mezzanines.  Safe Harbour got a lot of info about violence in the workplace!
Part of the idea is to eliminate dragging a portable stair around the vehicle during the work cycle.  The process usually takes at least six ascents and descents for one cycle so there is a lot of time to be saved as well as fatigue eliminated.  
The variation in height precludes a fixed catwalk as does the need to access the underside of the unit.
Brainstorms and mild mental disturbances welcome!

Griffy

Maybe G-raff has something:
http://www.g-raffsystems.com/G-Raff/elevated_platforms.html

Ted

Depending on the bay height and truss load capacity, I would investigate using suspended scaffolding like swing-stages arranged in a U-shape.

How about a lift? the drive on car lifts come to mind, just set it up wider and slide the cross piece to the front to make your U.
http://www.ben-pearson.com/images/14k-lift.jpg

Luck is a difficult thing to verify and therefore should be tested often. - Me

Hydtools,
G-Raff looks very promising.  The elevating platforms are very close to what we need.
Carnage1,
I looked at the car lifts and some styles show promise but others are unusable.  Biggest factor is the in-house engineering that would be needed.  Let someone else take care of the certification for OSHA if possible.
I knew someone out there on Eng-Tips would have some ideas.
Thanks for the help!  

Griffy