If the wheel seal is bad you will have oil in the brake drum (be sure to inspect the brakes for oil contamination). If the axle flange to wheel hub is leaking you will get oil on the outside of the hub (as you are reporting).
How long has it been since the axle shaft flange gasket was changed? If it was not long ago it is possible the mechanic used RTV to seal the flange but did not clean the surfaces the well - so its leaking now.
It is also possible that you have a pin hole or crack in the hub allowing the gear oil to seep out over time. Here's a video describing this situation
https://www.youtube.com/watch?v=H_b71Sgm2PsI would start by inspecting the brakes (just get under the coach and look at brakes (no need to disassemble anything) if it is dry, then move your inspection to the front of wheel hub. Clean everything with degreaser (can of BrakeClean) and wipe everything dry. It make a take a short drive (or couple of drives) to see where the oil is coming from if its a pin hole in the hub.