We have a 2014 HW-277. King bed out front. Our typical working load is about 450 to 475 pounds including mattress pad, electric blanket, several more blankets, and 4 pillows. We camp a LOT, so our rig has been out no fewer than 50 times...always boondocking on rough terrain.
We haven't experienced any signs of the bed being overloaded, even when dynamic loads are applied.
I have noticed that the doorway gap has opened up slightly at the top, which I presume is at least partially related to the bed leveraging that relatively unsupported sidewall a bit. On my rig, the door is only about 20" from the front wall of the camper....other campers' doorways are located in different spots depending on interior layout. My rig is wood-framed, so rigidity is not a strong point.
The pipe frames that support the bed ends show no signs of stress, but the weak point in the pipe supports is the hinge pivot between the horizontal and diagonal pipes. Watch that area for signs of fatigue or distortion.
On my rig, the lower, diagonal leg of the pipe frame can be nested in any one of 4 slots on the A-Frame or Rear Bumper. Choose your slot to avoid any noticeable sag in bed from where it slides out of the trailer body to outermost point. This ensures that the pipe frames are keeping the sliders properly aligned and that you minimize the leveraging on the PUP body...especially on the door side. The pipe frames carry the lion's share of the load, and the slides and aluminum bed frame only carry load at the point where they join the body....and add rigidity to the plywood platform.
P.S. Completely unrelated to weight rating, we found that the pipe frames are very attractive "nesting" areas for bugs...typically black (house) flies, but occasionally bees or wasps would move in. On some occasions, hundreds of flies would come out of the pipes when we broke down the camper. Spray foam works, but it's messy. I used household window and door caulk to plug the ends of the pipes. Big improvement, and it eliminates all that extra weight of flies in the pipes.