Those metal disks are forged as part of the hitch pin ... side view of the pin looks like this:
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The flat part of the trailer hitch should rest directly on the hitch skid plate, the flat plate of the hitch.
Try lowering the trailer down, making sure the top flange of the hitch (the "metal washer") settles into the hitch - not sitting on the skid plate. You may have to move the truck forwards or backwards while there is downward pressure on the hitch to get the top flange past the skid plate.
If you're lucky, the bottom flange will also slip past the locking bar so the bar can move into the pin's waist where it belongs. Then you should be able to pull the release handle to move the locking bar out of the way, lower the landing gear just enough to take some weight off of the truck and let the trailer slide out the rear of the hitch.
If not, and the release bar is still jammed against the bottom flange of the hitch pin, you'll have to pull hard on the handle and maybe hit it with a heavy hammer to get the locking bar to move to the unlocked position. If you have to resort to a hammer, I'd have a qualified person inspect the hitch for damage before reusing it.
My apologies if this is stating the obvious, but you don't lift a 5th wheel in and out of the hitch - it's supposed to slide out the back after you release the locking pin. Lift the trailer just enough to take the weight off of the truck so it'll slide easier.
Same thing when you hitch up - the trailer should be just high enough to contact the hitch just behind the midpoint of the saddle, so it rides up on the skid plate and levels it out against the trailer's plate before the pin slides into place.
When you hitched up on uneven ground, the pin wasn't fully down on the hitch - you "high pinned" so the locking bar went across the bottom flange instead of staying in the waist where it belongs.
Since you don't have a hitch that tilts from side to side, the way to avoid this problem in the future is to run the low side of the truck onto some leveling blocks so it's tilted in the same direction as the trailer.