Roof air conditioning will cool your RV down by 20 to 30 degrees. If you only have a dash air conditioner it will not cool down the area beyond the driver and passenger seat very well.
In order to run the roof air conditioner while traveling you must run the generator. You must remember though that these run off the same fuel as the engine, so it will utilize more fuel.
Many RVs have duel roof air conditioners that cool off the RV faster, and in order to turn on both air conditioners you must use the generator. If you have 30 amp service in your RV it won't have enough power to run them both when hooked up to shore power.
Roof unit air conditioners can be converted into heaters by installing heat strips to the units which will help take the chill out of the night air.
Having dual air conditioners is great, but you don't need to run both of them at one time. Running them independently will save energy and fuel.
