Jfet,
I built a rear door ramp for our 10,500 lb capacity car hauler/implement trailer back around 2001 I had that the trailer deck was 44" off the ground. I made a knuckle joint cable armsystem going from 6' high reinforced 2" X 2" X 1/4" wall square steel tubes slipped into 2-1/2" square welded on receivers roughly 1' from the rear of the trailer. The removable posts were anchored forward by removable steel cables going approx 8' forward to the trailers side rails welded on clevis. The knuckle was closer to full extension (open) when the ramp was on the ground leaving about 8" additional lowering for ground that was lower than the trailer's wheels. It would pull up (raise) the ramp to level or even up to the vertical position or any position by pulling on the knuckle joint with a cable on each side to a pulley yoke which was attached (driven) by a 12V 12,000 lb capacity electric purchased barrel winch that I already had. The whole cable system could have been a pulley/cable system also but I wanted the mechanical advantage that a knuckle joint gives.
The entire system was secured to the trailer using 1/2" and 5/8" hitch pins so if I wanted to remove any item it was simply pull the keeper clip and then pull the hitch pin. A couple seconds max!
The heaviest item I'd lifted with the ramp was a 5700 lb tractor with a front bucket that a friend bought cheap from a farmer down near Indianapolis which had a broken axle and couldn't be run. The farmer pushed it onto the ramp with his Bobcat but couldn't get it up onto the trailer bed. My ramp lifted it without any problem and then higher with a downward angle and a 2 ton comealong slid it right onto the bed so we could haul it the ~250 miles back here. Cabled it to a tree in his backyard with the ramp down and drove the trailer out from under it so he could repair the tractor. Worked really slick!
Just some food for thought as a possibility. I don't have any drawings of it as I built it by using my engineering knowledge and experience. Lots of combinations are possible also and as you have CAD, it will be easy for you.
