My RV is on the MB Sprinter chassis but is by Thor. I had the opposite failure, lost brake and turn signals on the rear. Still had clearance and running lights. Front lights worked normally.
My RV---The MB front lights are incandescent, both signal and marker. The motorhome rear lights are LED. Under the rear of the chassis there are 2 dummy load boxes (Matchbox size) to load the light circuits similar to incandescent bulbs. When my rear turn/brake lights failed it was a relay, not a fuse under the drivers seat that connected the coach to the chassis. Absent a wiring diagram from Coachmen, you might try making sure the relays are seated and tapping them to possibly get one to operate.
Good previous recommendation to check with a meter, start with checking for 12 volts for your taillights at the hitch connection.
And-- Welcome to IRV-2
Thor emailed wiring diagram the next day, hopefully Coachmen will respond.
Hooligan, Pensacola, Fl -U.S. Coast Guard 1956-1985
2016 Thor Siesta Sprinter 24ST diesel
Our Pug "Lily" & "George" the Newfoundland