Here's my trailer. It's really a car trailer I built, but I haul logs/firewood on it in long lengths. The first few times I hauled long lengths there was a tractor available at the site and we used that to load the logs.
The last few sites I didn't have a tractor with a frontend loader, so I started to devise some sort of loading system. The first thing I did was go to harbor freight and buy their 12000 winch on sale. That thing works really good so far. I then needed a way to get the logs up on the back of the trailer and to also load them on top of each other. The first version is shown below, it's a hinged bar with rollers on it. The height is adjustable by the winch mounted on the far side in the picture on one of the standards.
I did get the wood that is shown in the picture loaded with this first gizmo. The first problem I had, the first log I tried to load was a big one, and the little winch operating the bar would not pick it up. After that, the winch was strong enough for the rest of the wood, but there were other problems. Stobs or short broke-off branches on the bottom side of the logs would hang up on the bar/rollers. Also, I had several short rollers covering the bar, one long roller would have been better.
I didn't like the way it turned out, so the next plan I formulated was to build a sort of gantry crane off the standards with a long I-beam down the center up high, with a trolley. The trolley would have a pulley. The 12000lb winch cable would go through the trolley pulley and could be locked. The trolley could also be locked to the I-beam out at the tail end of the trailer. My plan was to move the trolley out to the tail and lock it. Then use the big winch to skid the log, and then let the log start going up to the trolley. Once it was high enough, I was going to lock the cable, but unlock the trolley, and let the winch pull the trolley/log up onto the trailer. I started on this idea, but what stopped me was the i-beam. It was very heavy, and the whole system was getting complicated. Whatever I build needs to come off without too much trouble, after all the trailer is a car trailer also.
What I have done now is to take the system I had in the picture above, and move the hinge point back to the very rear of the trailer and remounted it. I then made a wooden square frame that fit down inside the metal frame, and made it solid with with wood. It turned into sort of a wooden ramp, hinged dovetail that goes right to the ground to skid the logs up on the trailer. One other trick I am going to try, the wooden ramp is hinged to the metal frame. So after I get the first layer of logs on the trailer, I can hinge the wooden ramp up to lift the skidded logs a little higher hopefully over the first layer of logs already on the trailer. I have yet to try this latest version.