Yes.. with 30 amp service you have one 120 volt 30 amp line. With a 50 amp service you have two 120 volt 50 amp lines. For the record nothing in your rig runs on 220 volts. On a 50 amp service, half your rig will run on one 120 volt 50 amp line, and the other half will run on the other 120 volt 50 amp line. Typically your A/C's are each placed on a different line, along with a balance of the rest of your appliances/outlets. This is why when a rig needs 50 amps and a guest will say I have an adapter I can use 30 I try to explain to them that a 50 amp service supplies 3 and 1/3 times the power of a 30 amp service and half your rig will not operate on 120 volts. '
Depending on the size of the heater, yes you can run that on a 20 amp outlet. 20 amp is typically the highest rating on an outlet in a brick and mortar home. 20 amps X 120 volts means a heater of up to 2400 watts can be run on a 20 amp outlet. Know that many campgrounds will have more than one sight on the same line coming from the main campground panel. That 30 amp line at your sight may be on a combined 50 amp breaker, meaning, if you and your neighbor both run the max, the main breaker will blow, not at your campsight. Have I totally confused everyone ?