It could be that the door frame is slightly warped, putting a lot of friction on the side of the deadbolt.
The remedy might take two people. One person climbs in. The other person stays on the outside with the key. The inside person leans on the door, and the outside person carefully tries the key.
If that doesn't work, the inside person pulls on the door. The outside person pushes and tries the key.
Just for diagnostic purposes, did this happen suddenly? Or was the deadbolt always a little tricky?
I'm assuming that you have tried using mass quantities of WD-40, right?
Let us know how it goes -- good luck!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)