I just bought this lock. RVlock Keyless Systems - RVLock & CO, LLC
It is $200, you can get a $40 coupon code from their Facebook site.
A bit expensive, but I like the idea of not having to use keys when we have 5 or 6 of us at a campsite. No one needs to carry the keys, they just have to punch in a number.
Took me 10 minutes to install and program, no alterations needed at all.