Most campgrounds offer 50 amp, 30 amp and 20 amp service - necessary for todays RV'rs power needs (AC, Microwave, Electric Hot Water, Sat. etc.) If you are a heavy power user, always opt for the 30 amp service. The 30 amp to 20 amp adapter (available at RV Supply Companies) cannot handle heavy loads (overheats the connection point - trips the campground breaker.) 30 amp extension cords are also available at most RV supply stores in varying lengths. I installed an exterior 30 amp receptacle at home near my camper parking spot.
If you don't have an AC or many 110/120 appliances you should be ok with 20 amp service (using the adapter.) The interior lights, range fan, bath exhaust fan are 12 Volt components supplied by the 110/120 to 12 Volt Power Converter or on board external battery.