Appliance Service 120/240-volt 
The OLD still used but no longer installed Appliance Service
The 50-amp 120/240-volt 3 pole 3 wire Service

Not for RV

10-50R

The old 50-amp 3 pole 3 wire 120/240-volt outlet is used for appliances. A 50-amp Double Pole breaker with 2 Hot and 1 Neutral. NO GROUND

The 30-amp 120/240-volt 3 pole 3 wire service.

Not for RV

10-30R

 The old 30-amp 120/240-volt appliance outlet looks like this. A 30-amp DOUBLE pole breaker with 2 Hot and 1 Neutral. NO GROUND
Appliances with a combination 120/240-volt requirement MUST have a Neutral (White) wire for the return path of the current. The 240-volt is used to power the heating elements where there is no need for a Neutral but powering the motor and the controls with 120-volt a Neutral is REQUIRED.

The Neutral wire carries power. If there is a cable failure the Neutral may get the same voltage as the live line. If the neutral is connected to appliance grounding without being physically grounded, the failure situation will bring the voltage to the chassis of the appliance. Protective earth (Ground) wire should only carry current under fault conditions.

In the past the body of some ovens, stoves and clothes dryers were grounded through their neutral wires, as a measure to conserve copper during the Second World War. This is no longer permitted on NEW installation's the NEC code was changed in 1996.

3-wire 240-volt Dryer connection  4-wire 240-volt Dryer connection 

In the case of an Electric Water Heater there is no need for a Neutral but it must have a GROUND. The Heater Elements absorbs and uses ALL the energy. The Ground is VERY important in this service. In case of a short the current would quickly go straight to ground and hopefully trip out the 30 or the 50-amp breaker from overload.

NEW APPLIANCE SERVICE
Since the 1996 revision to the NEC, these are the only appliance receptacles PERMITTED in new constructions. The old 10-30 and 10-50 outlets are still used in most homes installed prior to this revision. Compare the outlets you are planning to use to these. 
Both of these are OK to use for an RV Service. 

14-50R

The 14-50 120/240-volt Service is used in many applications including appliances, temporary job sites, small Mobil Home Service and 50-amp RV outlets. It is 50-amp on each leg or 100-amp total at 120-volt. 

14-30R

The 14-30 120/240-volt Service replaced the old 10-30 appliance Service by adding the ground wire. This service can be used for your RV as long as you make an adapter. It is 30-amp on each leg or 60-amp total at 120-volt.

  No adapter is made or available to plug your RV into the OLD 50-amp 120/240-volt 3 pole 3 wire (10-50R) OR a 30-amp 120/240-volt 3 pole 3 wire (10-30R) APPLIANCE service.

Converting the OLD APPLIANCE (10-30 or 10-50) service to a 30-amp 120-volt 2 pole 3 wire RV service is simple. Remove the Double pole breaker and replace it with a Single pole 30-amp breaker. This service requires 3 wires 1 Black (Hot lead) 1 White (Neutral) and 1 Ground. One of the hot wires will have to be made into a Ground wire. Remove the insulation or use green tape to identify it for future reference. Install a TT-30R receptacle and its ready to go.

Converting the OLD APPLIANCE 10-50R (50-amp) 120/240-volt  Service to a 50-amp RV friendly Service is not difficult. Since these appliance services usually comes with 3 wires an extra wire for the ground must be pulled for the 50-amp 120/240-volt 3 pole 4 wire grounding Service. The 50-amp breaker is the same, install a new 14-50R 3 pole 4 wire grounding receptacle and the extra wire. 

Converting the OLD APPLIANCE 10-30R ( 30-amp) 120/240-volt Service to a 50-amp RV outlet the wire size must meet the 50-amp requirements. Replace the 30-amp double pole breaker with a 50-amp double pole breaker. Follow the direction above for the rest. Both of the above 50-amp conversions will supply 2 - 50-amp lines or 12,000 watt power to the RV.

 If the 30-amp 120/240-volt OLD APPLIANCE  Service wire size doesn't meet the 50-amp wire size ALL new wires must be pulled or make it into a 30-amp 120/240-volt service. This is the SAME service as the 50-amp Service above with 4 wires except it will have 2 - 30-amp lines or 7200-watt power to the RV. This service would be preferred in a 50-amp coach to the single line 120-volt 30-amp service. Use the 30-amp Double pole breaker already installed. 

14-30R Install a new 14-30R 3 pole 4 wire grounding receptacle and the extra Ground wire. A home made dog bone can be made with a 14-30 PLUG and a 14-50 receptacle in order to use the 50-amp RV cord.  
ALWAYS check out new installations. Anyone can and will make a mistake.
Suggestions have been made that we can use the 10-30R or a 10-50R APPLIANCE outlet without modifications since the white is connected to the ground in the main box. Just make an adapter and use one leg of the 120-volt and the white Neutral wire. I personally wouldn't plug my RV into ANY SERVICE without a correctly wired Ground wire present. As to the pluses and minuses to this setup I will let others argue the Technical points.