Is it OK to use the Earth Post and Positive post on the right side of the engine bay to trickle charge the battery ? - 2014 E550 Cab

TIA

If you're referring to the battery connections described in your owner's manual as the jump starting terminals, the answer is yes -- connect your maintainer there. If your battery maintainer came with a short pigtail cable -- eyelet terminals on one end and a polarized connector on the other end -- it's well worth the few extra minutes to install it, as it ensures you'll not accidentally connect your maintainer backwards.

Thanks !

Reading the owners manual, it leaves me wondering. I will be leaving my car for about 7 weeks and am debating putting a trickle charger on it, or disconnecting the battery. The owners manual implies these chargers be connected at the normal jump start terminals.
So - questions for the group:
1- DIsconnect the battery for the 7 weeks, then charge the battery and hook it up again?
2- Connect my NOCO Genius maintainer directlly to the battery via pigtails?
3- Some internet research shows a second battery in some models. My 2016 E400 does not seem to have this, is this restricted to earlier year cars?
TIA

Attaching a battery maintainer is usually easier than disconnecting the battery. I store one of my cars for 5 months over the winter season, and leave a battery maintainer connected to it throughout.

^ x2

The AUX battery should be in the boot, rear right.

Check this (vid demo's 2014, but that should be the same for 15-16):

Thanks for that picture. On other MB boards they advocate putting the trickle charger on the aux battery vice the starter battery. Your thoughts?

Usually the aux battery is charged from the main battery management system, but you should check with someone in the dealer's service department who knows the details for your specific vehicle.

ISTR, the charging posts in the front will ensure the starter and aux batteries are maintained.

AGM(absorbent glass matting) batteries, do not like to be overcharged. If you put it on a trickle charger, it should have a fully charged sensor, to reduce the charge to a fraction of a volt.
These new cars, say 2020 or newer, have a lot of sensors that are draining the battery, even in sleep mode. So extended non use without charging, could discharge the battery beyond its ability to run all those sensors, let alone crank over the car. Low voltage, and zero voltage on these cars, can trigger a lot of fault codes, and be very disruptive.
My last trip to the dealership revealed a lot of the showroom cars were on trickle chargers.

A quality battery maintainer -- Noco, Ctek, etc. -- will have an AGM charge selection setting to ensure proper charging.