Sounds strange to me too. It is pretty common knowledge that RV keys/locks are not the most secure in the industry. Plus most folks have master keys...repair facilities, etc.
Here is a simple trick to see if your doors are being opened by someone other than you....take a small piece of scotch tape and put it in a location (top of the door) which wouldn't be easily noticed. The tape should be placed over the seal.
If the tape is broken or disturbed, someone is getting in. If it stays secure, and you still see changes inside the rig, you need a ghostbuster.