I found the "12.75" and ".5 f/r diff on another Benz site. Manual only appears to have data for using alignment tools. (As if DIYers have them). I don't know if the number is accurate. I was hoping a pro would confirm or correct me.

I don't suppose there are any electronics involved with this system eh? no switches, relays, fuses?
___None of the above. Only hydraulic pressure (with nitrogen gas in accumulators).

I'm not really sure how to test to see if the pump is actually running.
___Fluid will flow through resevour. Disconnect return hose from resevour, and continually add new fluid to flush system. It self purges any air in lines.

I don't know if the valve body is good or not ( are they really prone to failure? any other way to test?)
___Manual has procedures for presssure testing and as overnight procedure to ensure it's holding pressure. "Level controller" has very good rep for longevity.

but even so I'd still expect to hear the pump go under load.
___It uses the Power Steering pump _ but two different fluids are used!!! Because Power Steering pump is runnig constantly, I doubt you'd hear any different sound under load. Rear height should change within moments of turning on motor.

any other ideas on where to go next?
___We're both in the same wagon. Ride height of mine is similar to yours with little change loaded. I felt the handling was better when my accumulators weren't working properly. The handling of mine feels worse with the rear so high.

Do you mean the owners manual? I'm gonna go look right now. I've just gotta get this thing to level itself.