I just checked my Marinco catalog and my other suppliers. It seems that there is not an adapter as you specify being made now. You can get a 50-amp/50/50 'Y', or a 50-amp/30/30 'Y' as well, but nothing that goes 30- amp/15/15.
Remember that 50 amp park power is 240VAC with 2 hot legs and 30 amp is 120VAC with one hot leg.
It's not difficult to make one, but why not get a 30 amp/20 amp plug/socket and then use a 15amp/15amp 'Y' into that, or just use an outlet strip. Same results!