Answer: The basic rule is that a landlord is required to provide an RV park that is “kept in every part safe for normal and reasonably foreseeable uses.” Practically speaking, this means that if you know about a particular danger or threat of danger, as a landlord you must take reasonable steps to reduce or eliminate that danger.
The real question of course is what are the “reasonable steps” to take? At the basic level, you should make sure that you have adequate lighting that is well-maintained to illuminate the park streets at night (i.e., replace those burned out bulbs). Your resident manager should also conduct several walk-throughs throughout the day, preferably in the early morning and evening hours to note any problems. If you have a perimeter fence around your park, make sure that it is kept in good repair and that entrance gates are working and secure.
Part of the obligation to keep your park safe is to also promptly enforce all park rules with written warnings and/or eviction notices if necessary. A good example would be an unauthorized occupant staying with one of your tenants who you have reason to suspect is involved in park thefts. Even if you don’t have solid proof of the thefts, you can rely on the fact that the person is not an authorized tenant and evict on that basis alone. You should also immediately evict any particular tenant who is causing trouble. (Hint: Consider a simple no-cause eviction notice.)
If you’ve done all these things and are still having problems, do you have to hire a security guard? Probably not, but it all depends on the circumstances. The test is what is reasonable to eliminate or reduce the danger. In some cases, that might mean installing security cameras if it would reduce thefts and it is financially reasonable (and in this day and age, security cameras are relatively inexpensive and easy to install). In other cases, it might be reasonable to install a perimeter fence to keep out pedestrians cutting through the park (and helping themselves to tenants’ personal property).
In the end, you will be held to the standard of providing a reasonably safe park for your tenants. If you know about a particular danger or potential danger, take immediate steps to reduce that danger. While you can’t eliminate every threat, you can reduce your risk of liability by addressing foreseeable threats.