You must have the Trimark lock with the pot-metal latch that breaks and won't retract - common problem; plenty of discussion about it here on this forum (as well as many other RV forums).
Disassemble the lock plate from the inside. Look at the latch tha moves back and forth to keep the door closed (not the deadbolt latch). Use a screwdriver, pliers, whatever it takes to retract the broken latch out of the door jamb.
Buy a new lock and replace it. Some say you can call Trimark and they will send you a new lock for free, some have not been successful with that approach.
You can also get an aftermarket repair kit:
Thor Forums
