Hi. Have a 2014 E350 with 124,000km and noticed an issue with the fuel economy after the spark plugs were changed. They were bosch spark plugs do not sure what is wrong. Any ideas? Thanks

Are all the plug wires and coils properly connected?
Are you sure?
Who did the spark plug replacement?
Was a torque wrench used when the plugs were installed? Modern spark plugs need to be installed with the proper orientation for best performance, this is accomplished using a torque wrench.
Do you have a check engine light illuminated? Have the codes been read?

How did the weather change in your area in last weeks.
I was still using my swimming pool 2 weeks ago, but now we have view of beautiful snow on mountains across the valley.
Lower temperatures will affect your mpg, especially on short commute.

Gas stations are switching over to winter blend. In Chicago some switched over almost a month ago, while others were just switching to winter blend this week.

There's no way a new set of plugs alone made a noticeable difference to your fuel economy, as above i'd suggest the fuel or weather might be the cause.

Is 2014 a direct injection engine? If it is I think the plugs need to be indexed so that the electrode points in correct direction inside the cylinder. This has to do with how well the spark can lit up the fuel mixture. If wrong plugs are used in DI engine they can cause loss of fuel mileage.

The 2014 E350 W212 used the M276 direct injected gasoline V6.

I changed it recently, about 2 weeks ago? Maybe it is the winter fuel. But I don't know if this is just me, but every time I start the car, I find the fuel gauge gets a bit lower

Do you smell any gas on start up ? or whenever ?

Vacuum the car carpets and you notice on next start it will be higher.
GUARANTEE..

You are correct. Have to index the plugs. That is why MB plugs have threads that all start in the correct orientation. Then torque to right specs.