Assuming you have a 50 to 30 amp adapter so you can plug in to 30 amps (similar if all you have is 20 amps), it still depends on how your RV is set up. If it has an EMS (Energy Management System), then some things (air conditioners, washer, etc.) will get disabled if the total load gets too high. Otherwise, you'll need to be careful not to run too many things at once, especially air conditioners, microwave, washer/dryer and other high current devices, else you'll probably pop a breaker.
Purchase a 50amp to 30amp Adapter. This adapter will supply the single 120vac 30 amp leg to both legs of your 220 vac 50amp power cord. Your EMS will sense that your input voltage is not 220vac, hence 30amp service, and will start shedding loads.
With or without an EMS you will be limited in the number of devices you can run simultaneously (remember that your converter is always connected so don't ignore its load when determining what devices you can run).
We are currently in that situation. For us it simply means that the A/C and electric water heater don't get used at the same time. Also instead of being able to run two space heaters (on different circuits) when it gets cold, we'll only be able to run one.
In addition we also run the fridge on propane only to eliminate any risk of a voltage drop damaging circuitry on the fridge. Since we don't have an EMS on our rig it just means that we need to pay a little more attention to detail when it comes to electricity. If I had my druthers I'd be on 50amp, but sometimes it just doesn't work out that way.
When I park my 50 amp motor home on a 30 amp site (Such as right now) I either use a 30 amp "Dogbone" type adapter (This is a short cord with a 30 amp plug on one end and a 50 amp outlet on the other) or I use the 30 amp shore cord (this is a cord that has a 30 amp plug on one end and a 50 amp Marinco outlet on the other matching the inlet on the motor home)
You can get the adapters, or a short (1') 30 amp shore cord at Camping World and selected RV dealers.. You can get the Dogbones at Camping World, All RV dealers, Wal-Mart, (And some other chain stores) and many campground camp-stores.
Dog bone as mentioned above and REMEMBER that you have WAY less electricity than on 50 amp. So turn the water heater on and off as needed. Turn the AC on and off, the microwave and one other item at the same time may be enough to trip the breaker. It can be frustrating, but it can be dealt with. We have a rule that we turn on the Hydro Hot for hot water and leave the electric water heat off. We run only the AC, Microwave, hair dryer, curling iron or griddle one thing at a time. Sometimes...... one of us forgets. So, after the second or third time one of us forgets, we have a pow-wow at the breaker box and both of us learn how to reset the breaker. For some reason we tend to remember better when the one responsible for blowing the breaker has to put their shoes on and reset it. I love you Honey.