As with most campers, they are pretty easy to get into if someone wants in. Yes, you can pull up the canvas from the velcro or, just use a knife.
When we had our P/U, we never kept anything of any real value inside the P/U when we weren't around. Put everything of value in the TV and locked it up. Keep everything in bins so no one could see what was in them.
Usually, you will find that campers look out for other campers, but there is always those who are looking for an easy score.
2016 Forest River Surveyor 201RBS
2014 Chevy Silverado 1500 4.3L