A lot of things in successful animal training are to do with choices. I'm sorry if that sounds flippant, but bear with me. I'm old and work alone so my ponies have to learn to self load. I refuse to go into a confined space with an animal that weighs over half a tonne, and is probably upset, especially as my mobile is in the house with a flat battery!
A round pen is a great asset thought I'm told a lunge line will work. I haven't tried it and suspect it isn't as good. In the absence of my pen, I think I'd rig up a makeshift pen with some tape. My method is very simple. If they are outside the trailer, they work. The nearer they are to going in, the less work they do -- or the better the rest. Keep everything calm with no force. If they go in and immediately come out, that's fine too -- more work! It's their choice.
So, they get worked and eventually will put a foot on the ramp. I immediately turn my back and walk away and stand with my back to the horse for maybe a minute. Then work starts again and I demand an inch or two more each time. Hunger helps as there is a full hay net in the trailer. Training being what it is, it won't be a steady progression but more like three steps forward and two back.
This is where choices come in. Outside the trailer, the horse works. Completely inside, I leave the horse to its hay net and go and have a cup of tea. If they want to come out, that's their choice but, guess what? More work!

I'll continue until the pony loads without hesitation.
Of course, the above is similar to several NH training methods but the penny dropped when I had a very shy mare in for service owned by a no nonsense livestock haulier. I've never seen an animal load as fast in my life! It was easy to guess what had been going on and to use a bit of imagination to vary the technique to be totally humane but every bit as successful.