My braking was noticible weaker a couple years ago. Adjusted the brakes put them back like new. 
If you want to check them yourself, jack one wheel slightly off the ground. Pull the break away switch and try to rotate the wheel. It should lock up if the brake is working electrically. Put the breakaway switch back in. Using a cheap brake adjustment tool from auto parts store, adjust that brake. (Google RV brake adjustment for info and videos). Lower wheel and repeat with other wheels. Pretty simple. 
If one or more does not work when the break away switch is pulled, then it will take some troubleshooting to find out why.
However, depending on the sound when you "hear them", it could be the brake linings are worn down and need new brake shoes.