I have found that most of the mud flaps made to fit around the fender wells work well to protect the truck but leave enough room between the flap and the ground to allow enough mud/dirt/rocks to hit the trailer. I installed a set of hitch mounted mud flaps on my last 2 trucks and find they work better in protecting the front of the trailer. I set the height so the flaps are closer to the ground then the fender flaps but still clear on dips so they don't drag. And I can remove them easily if not needed.
There are many brands out there now, like Road Wings, Rock Tamers, Smart Solutions, etc. I have Road Wings.
And there are many places to find them like truck accessory stores. Also Amazon has many choices as does Ebay. Even Cabelas has their own brand. I recommend you check these out. If you get a set purchase them with the solid flaps, not the style that looks like a broom. Those flap in the wind too much.
I also have a full flap on the rear of my 5th wheel to protect the boat I sometimes pull along. It also helps protect the boat/trailer.
