I know this is an old thread, but someone else will also have this problem, so here is what I found on my 2019 Forest River MBS (2018 MB chassis)
My trailer 7 pin connector on back was installed by Forest River. You can tell by just looking at the install. It is all wrapped up with electrical tape and it is wired into this black box mounted to a board with power resisters on it that controls the rear light module. This module was just kind of hanging there tucked up between some spray foam and rear wall of RV.)
I did find a blue wire that ran from this Forest River connector up to under the drivers side seat with a crimped on wire nut on end. Also under this seat is a factory connector for the trailer brake controller, but it did not have power on the wire that supplies power to the brake controller.
I was trying to figure out why the factory brake controller connector under the seat did not have power:
I called Forest River and they denied they ever even ran a blue cable from the 7 pin up to under the seat with some excuse that MB does not let us attach to any wires in the factory harness
I called the dealer support line and they knew even less.
I called MB dealer, they could not help me with my power problem but they said "your VIN indicates your RV has a factory installed 7 pin wire harness".
I called prodigy brake controller company and they said "I bet you have to plug in a trailer before that connector will show power on the brake controller feed wire".
I plugged a trailer into my Forest river 7 pin connector, still no power to the MB brake controller wire.
Looking under your RV on Drivers Side, behind the rear axle, around where the factory frame ends and the Forest River extension is bolted on. I found the MB factory 7 pin connector zip tied up next to the frame covered with spray foam.
I removed the wire ties and foam, bought a mounting bracket and mounted the factory connector under RV.
I removed the Forest River supplied connector from its mounting bracket on hitch and zip tied it up to frame area.
I used a 4' - 7pin extension cable (I had from my truck camper hitch extension). I mounted this extension cable to the bracket on back that Forest River had supplied and plugged other end of extension cable into the MB 7 pin connector i mounted above.
Plugged in the trailer and lights work perfect, and now I have power on the brake controller feed wire that is in the factory harness under the seat
I bought a prodigy brake controller and the wire harness adapter cable they sell for the Mercedes sprinter. Plug one end of this cable in under the seat and the other end into the brake controller that I mounted under dash.
now I have the Factory 7 pin connector including any special software MB might have to control trailer brakes associated with the ABS system.