You could always go to the RV place close to your location. They would have a door holder, which is a T shaped pin that hooks into a clip you would install on your door.
As for location, I think you need to look at another Tril or open the door and see what part of the door meets with the trailer. You want it where the door and trailer will touch. So on a Boler/Trillium, since the top and bottoms curve inwards, your door may be farther out at the top and bottom when open.
On my Cadet
it is pretty much in the middle.