Your tail lights and Brake/Turn are probably in the same housing, so the fact that the Brake lights work indicates the ground is good to those assemblies.
It is likely that the marker lights are tied into a single wire run
coming from the front of the coach, the wire runs to the rear. Somewhere between the front and the middle, the connection is broken.
Are the marker lights on both sides not working??
I doubt that you have two marker light fuses.
Something to try.
Disconnect the chassis batteries if you like, or just be sure the headlight switch is OFF.
Remove one of the non-working marker light lens.
Remove the bulb.
Determine which socket contact is ground...although this is not critical.
Using a 12v lantern battery, connect the +12 & ground from the lantern battery to the socket contacts.
I suspect your non-working lights will light....and the good ones will not light up because of the broken wire (or bad connection).
2014 40QBH Phaeton DP Cummins 380HP ISL, Freightliner XC, 2014 Jeep Trailhawk TOAD
Blue Ox Aventa tow bar, SMI Air Force One -South Orange County Ca.