They hop into the dark horse trailer w/o issue. Which is clearly visible in the background.Do you have a flat bed trailer that you can use for training purposes? It might help them to learn to just go up the steps and not have to worry about entering a "dark cave" for starters. Just thinking out loud - you'll probably get 'em in pretty quickly either way.
It used to be a very dark blue on the inside. Painted it white, but not before they went in a few times.That's great! And, yes the trailer is clearly visible but it is light colored as opposed to your black van. Sorry - was just offering an idea - but you got this!