D Maxer, The common lock fitted to RV's is Camec - they invariably fail sometime locking owners out of their vans - there are a lot of videos on U Tube related to their problems.
I have done a C&P of problem from another site;

1) After doing a major dismantle of the door, found that to replace the door lock barrel is simple.
Place the key in the lock barrel & with the lock tabs in the down position, use a small screwdriver to depress the locking tab on the barrel, & slide the assembly in until the locking tab locates.
I dont know why the barrel came out in the first place, but its still there & the problem has not re-occurred again.

2) After removing the door handles, they have to go back in.
After much trial & error, this is how you do it:
Place the outside handle on first, unlock the inside handle, & place it where it should go (except it wont).

Enlist the assistance of your beloved or one of the assembled crowd of rubbernecking sightseers.
Get them to depress the bottom door latch, while you depress the centre latch while pushing the handle back in place.

The bottom latch lifts a spring-loaded bar that operates the latches, & the centre latch engages a rack on the metal bar, with a pinion on the handle assembly.
These need to align, before the sliding aluminium actuator of the handle assembly will insert into a slot on the latch bar.
Replace the top & bottom screws on the door handle. You need to lock the door from inside to get access to the remaining screw. (press the spring loaded button near the latch to be able to lock the door while its still open)

Sounds complicated? Not really, patience & common sense are all thats needed. (Dont take too much notice from all the uneducated spectators.)

3) Now the funny part, of fixing the locked-in situation.
I waited until we got home to dismantle the door bits.
I completely dismantled the door handle assembly, & found an absence of any sensible reason why we got locked in.
I did find a slight burr on the sliding aluminium bits, which could catch, but I dont think that this was the cause of the problem.(removed the burr anyhow)
I thoroughly cleaned all parts & lubricated everything with silicone spray lubricant. (Dont know if this is the best procedure, but time will tell.)
Thoroughly cleaned all bits in the door, & using previously gained experience, reassembled the door handles into the door.
I adjusted the striker plates to where it seemed to be a good place. (It would be nice to know the exact procedure.)

I also adjusted the striker plates on the screen door to make it unlatch easier.

Everything back together again & the door locks are working like a bought one!

I am hoping not to have to go down that road Possum. At this stage the door lock still works but only if the safety button is depressed manually. 

It is one of those rubbish plastic ones that come with all caravans Tony Bev. It is a Camec three way lock.

I have just been given some brilliant advice by one of the locals who had similar problems with his safety button not engaging properly with the striker plate. He suggested putting a little touch of paint or nail polish on the safety button and closing the door. This in turn will leave a little raised film on the striker plate and will therefore fix the problem.

Will keep you posted. Thanks for the ideas so far gents.

DMaxer wrote:

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I have just been given some brilliant advice by one of the locals who had similar problems with his safety button not engaging properly with the striker plate. He suggested putting a little touch of paint or nail polish on the safety button and closing the door. This in turn will leave a little raised film on the striker plate and will therefore fix the problem.

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 I would not disable that little button and negate its action. Way back we had the Trimatic doors. The Trimatic Door Lock was the one with a cogwheel as the striker. In the early models of the lock, the cogwheel would turn when the snib on the inside was pushed down. When this happened the door would lock and you could not get back in. Most of us experienced that happen, mostly with the keys in the van. I replaced mine with the later model lock that prevented this from happening.

I suggest that instead of fiddling with the action of the safety button you adjust the lock striker plate and/or the door frame. The striker plate has enough movement to allow that button to be pushed right in. If the problem is that the door frame is too far away from the door for the safety button to contact it, the door frame will have to be removed and sufficient packing inserted between the door frame and the cutout in the van. The inside trim just pulls out, you may then be able to slacken the screws that hold the door frame in and pack the frame out enough to contact the button. Then replace the trim and tighten the screws. The locks are a bit fiddly to adjust when things get out of place but if you persist you should be able to get it working properly.

If the button is over the door frame then just the striker plate adjustment will fix it.

The frame the door swings in.