this is what the 50 amp service looks like in my cardnal. the bar across the two 50 amp mains means they work together ie one side throws the other throws. it is made for a 220 volt input. when u adapt to a 30 amp service, it supplies both legs with 110 from the same campground breaker. right now, there is nothing on 220 in the trailer. in the future, there could be.
where u can get into a problem with this is using one of those boxes that pluggs into two campground outlets that supplies each leg in the trailer with its own 110v. if the voltage is of the same phase, the neutral has to carry the amps from both legs back. if they are out of phase (220 between the legs), the neutral carries current only when there is an imbalance.
the 4th wire is a ground which is isolated from the neutral in the trailer (in the home, they come off the same point). the extra wire is needed because the trailer is considered a sub panel.
its like ur home in that 220 is supplied to ur house but it is seperated into 2 110 circuits.
basicly, the explamations above are valid. what has the potential of getting u in trouble is one of those 50 adapters that uses 2 sources. if u plug one of these adapters into a 20 amp (breaker protected) service and the other into a 30 amp (breaker protected) service, u will be ok. i have one of those adapters pictured below.