Two same answers:
1: If you are having a proper RV outlet installed. go with 50 Reasons are two fold
1A: Many times I have read (and once prevented) of an electrican wiring a TT-30 for 240 volts... Now that one the customer (And electrician) got lucky cause I got to him in time (While he was purchasing the outlet).
1B the cost of upgrading from 30 to 50 is far more than the difference.
Now in yoru case
At the RV end you can either install a standard Park type box (20-30-50) or you can use the 50 amp splitter This splits a 50 amp into two 30's Or you can make a 50 amp "Splitter" that feeds one leg to the RV via a 30 amp breker and the other lead is split between a pair of 15/20 Duplex via breakers for each (how I'd recommend doing it)
Or make up a 30 to 15/20 box with the proper 15 amp breakers
use this for the Outdoor Electric stuff like the grill.