You can't get an RV with windows too small to climb through unless you modify them yourself. However, you better sleep in a fire suit because if you're in the back bedroom and the front of the RV catches on fire for some reason, you'll have to run through that fire to get out. The best bet for you is some type of security system.
One thing I can tell you for a fact, is it is generally much easier to break through the front door of a regular house than it is to break open a locked door on a RV. I know this because I've done it quite a few times when I was working narcotics and serving search warrants. Whenever we searched an RV or trailer getting through that door was not an easy task because they are a little better built than wood house doors and they open outwards. So it could take up to 5 seconds to get them open as opposed to the 1.5 seconds it took to get through a regular front door.