We live in Florida and camp in the Keys and elsewhere for short trips year round, sometimes with our young niece. On some clear summer days when camped with little shade and very high humidity (a not-uncommon condition in the Keys and beach sites), the 13.5 AC unit in our prior 2011 25' FC twin (single seaming) could not keep up. This was true even with every window and vent covered inside with Reflectix, and the trailer (cave?) well-cooled early in the morning, with fans at both ends of the trailer. (On one particularly bad occasion, inside temperatures gradually climbed into the low 90s.)
We had that AC unit tested by two different RV dealers, and were told both times it was operating within spec.
We traded that 2011 FC in for a 2015 27' twin EB unit with 50 amps, two ACs (one 15.0, one 13.5), awnings all around, and clear 3M heat rejection film on all windows. We may have gone overboard with all this, but we now head into the hottest summer weather confident that we will not have an issue with inside temperatures.
We have not found any major downsides to having a 50 amp system, and don't view hauling a heavier cord and heavier trailer weight as big issues given the benefits of 50 amps.
It is possible that a single 15K AC, awnings all around, and 3M film alone (without the second AC) could adequately address the issue we faced, but we did not want to take a chance, we opted for AC #2, and we are glad we did.