If your guy can’t install a simple three-wire, 30-amp circuit, I’d be hesitant to have him install a four-wire, 50-amp circuit! It’s probably best to contact a local, licensed electrician who also understands recreation vehicles.
Personally, I’d opt for a dedicated, 50-amp receptacle, (NEMA 14-50R), and like you suggested, use an adapter for your current motorhome. Better to be equipped for the higher current/larger conductors in case you were to ever upgrade your RV to one with 50-amp service. If you install only a 30-amp receptacle and then buy a new coach with 50-amp circuits, when you adapt the other way (30-50), through the adapter, you'll lose all the circuits on one of the hot legs inside the coach. Better to have the higher amperage receptacle and be set.