This is how i fixed the flimsy doorknob on my Clipper 17BH but can be used for any other camper that has the same problem.
See, the way they make the doors to be light weight makes them also very weak when it comes to the doorknob being tight enough to not move around.
The way i fixed this is by inserting small blocks of wood (i used pine) and glued them around the area where the two screws for the knobs are. First you need to remove a bit of the inside foam tho. Also, make sure you can insert the pin back in its hole before the glue sets in.