Hope you're not driving to Alaska. When we were in Whitehorse Yukon, people were removing the big stiff flaps saying they were throwing rocks on their windshield. We went over to their towed and there was a layer of 1/2" rocks laying at the base of their windshields and the hoods were damaged.
We had just come up the same road behind them. We had flaps behind the tires but not across the rear. We didn't get any damage to our towed - there, and for the whole summer trip. In fact, in 8 years we received no damage.
A few months later we were traveling on an interstate in heavy rain. I made it a point to watch motorhomes going in the opposite direction. Those with the across-the-rear flaps had the water going high on the toad's hood. Those without the flaps had the water at tire level.