[JRTJH]

Possibly the striker plate needs to be adjusted.

As for having to slam the door to close it, if your trailer is relatively new, chances are that it's "air tight" and you're "fighting the pressure" when you close the door. To find out for sure, open a window next to the door, then go outside and see if it closes with less "need to slam".... If so, that's a good thing, you aren't losing inside air which should make your trailer cooler in summer and warmer in winter (without the air leaks)......