Your original question is unclear, as others have said. If indeed you are trying to get 50 amp service off of shore power poles that have only a conventional 15-20 amp household plug and a single 30 amp plug you might consider one of these:
Amazon linkI have gotten an earlier version of this to work, but it seems to work best using two power poles if you have a vacant site near you, using a 30 amp extension cord to the other shore power pole. For those of you who might think I was "stealing" power, I was on a daily rate which included all electrical consumption. I have also tried this on a single shore power pole using a 30 amp female to 15 amp male adapter. It didn't work as well and tended to pop breakers.
It is certainly an adaptation and it's better to just have a 50 amp source.