appears to be a 21,000 pound trailer, truck most likely rated at 25,000 or thereabouts although being a dodge I'd be it doesn't have a GCWR sticker, so you can totally legally get away with 21K + whatever the truck axles are rated at, probably 35K gross
I'd load it as heavy as needed and just go slow enough for the exhaust brake to handle all the work down grade and be 100% comfortable, my dump truck needs an exhaust brake in a bad way, we see 25% grades all the time, truckers always mention the 6-8% grades on the interstate but nobody talks about residential and backroads, the road infront of my house is 28% and its not even bad for my area, pretty common for me to drop down into first gear before a hill and still have to stand on the brakes to hold it back at 3mph all the way down 1/4-1 mile hill