We have several documents on that keyless entry, at
this link.
Changing the code generally requires that you know the current code. If you don't know it, you may have to remove the window from the door to gain access to the receiver and start over.
If the device has been working and recently stopped, it may need a new battery on the transmitter. Check after dark. If the battery is ok, you'll see the buttons illuminate as you entry the code sequence to unlock the door. It's not very bright, so it really has to be checked in the dark. If the buttons don't light up, either the battery is dead or the device has failed. The keypad has to be removed from the side wall because the battery access is on the back of the device. It can be re-attached with double-sided fender tape from an auto parts store.
Also, clean the contacts on the inside of the door frame. If they get dirty, and they do, the receiver doesn't get any power.
If you can't get it working, I'd suggest you consider upgrading to the RVLock 4.0 device. There's a special running through the end of this month.
Here's a link.