If all that is needed is a couple of extra outlets in the trailer, then the 20A addition is the cheapest and easiest way to go. The advantage to adding a second 30A service is that you can sort out the loads according to current usage, split them according to current draw and get a balance between the two panels. This is easily done using a breakout box and running off the 50A outlet at the pedestal. That doubles your available service to 60A. If you do that and add an additional inlet to be fed from the pedestal's 20A outlet, that gives you a total of 80A of available service. Not as much as 50A service, but it's enough to satisfy most camper's needs.