There are thousands of RV's flying, that have entirely acceptable engine temps (CHT, Oil, etc.), even in hot climates.
When temps are not acceptable, there are two basic possible causes.
1. There is not enough cooling airflow available for the specific engine installation.
2. The cooling air that is available is not being fully/properly utilized.
Over the years I have had the opportunity to look at a lot of engine installations in RV's. If it has been to help someone diagnose a cooling problem, the majority of the time it is resolved by fixing problems resulting from cause # 2.
It has been extremely rare to see one that couldn't have some level of improvement in the baffling/cooling system. Countless times I have helped people that were adamant that their installation was as good as it could be done, but it was far from it.
Adding cowl flaps to improve cooling, before doing a very detailed analysis of problems that may exist in the entire system, is similar to installing a fifth leg on a wobbly 4 legged table.
This is not meant as a cut down to those that have installed them (or manufacture them). There are instances that it is probably the only remedy (engine modifications producing more power, etc.), but the number of RV's flying successfully without cooling mods is evidence that in most instances it is probably a short cut method to resolve a problem.
My opinion anyway....