My Dead bolt lock would not lock all the way when we first got our trailer. We had to drill out the opening where the bolt went into.
However we do not have any problem locking our door when connected or not connected to the TV.
That would indicate to me to look at how much the frame is flexing.
I do lock the main door to our trailer when traveling. We keep our Yorkie in the TV with us so that is not a problem. When we first started to travel fulltime I read a thread about someone that was going through a town and when they stopped at a place to go in and get something that someone opened their trailer got in and road around. Got out when they stopped at a light.
BC