FYI, Just be aware:
Don't know what type of door lock you have but what you described is usually the beginning symptoms of the lock itself failing. Not from lack of tightening/lubricating but from the pot metal part inside failing.
When the part finally breaks you sadly 'will' find yourself locked when the key can't turn it.
Had it not only happen once but twice. Got locked out and in of my coach both times. First time cost me $500 to get the door open and ordered new lock while on the road in a CG.
The second time I got locked inside and I was so mad I ripped the dang thing apart my self and disengaged the part of the lock that had the pot metal mechanism. Leaving only the dead bolt part of the lock working.