This may or may not be your problem since my experience is with a class A, however, they both have similar style doors.
I need to add this to my Why didn't anyone tell me thread.
I learned, perhaps here, that the entry door will have two modes of shutting. A light shut that is sufficient for going in and out all day and a hard shut which locks it in better for travel.
I was told that if you have to pull on the door to lock it then you are not in the locked position for travel, I was always doing this. I was afraid to slam the door but had to try. It took a couple tries only because I feared shutting the door too hard.
Well, whadda ya know, it worked. On my successful shut with the right amount of force on the shut and it didn't back off that little bit it used to. Now for the test, I turned the lock and it slid smoothly. Now for the final test, driving, it was secured in the locked travel position and no longer jiggled in place.