Wiring 50A RV receptacle to 120V service
No.  You cannot hardwire a 50A receptacle onto a 30A breaker.   

Given your goals (30A single-pole breaker) use either a TT30 receptacle or a NEMA L5-30R.   

You are welcome to fabricate your own dogbone adapter, with a NEMA 10-50R on one end and either a TT30 plug or L5-30P on the other.  

Inside the homebrew dogbone adapter, I am OK with hooking both 10-50R hots to the same supply hot, but: Be very careful that your RV is not wired with any multi-wire branch circuits.  Those will overload the neutral wire in that configuration. Because of this risk in using the adapter in random unchecked RVs, be very careful who you loan that adapter to.