The soil you are parked on is the first place to start. Don't try to level on clay side hills or gravel as both shift under weight and rain will make clay into a slip and slide. Sand is easiest as is silt which is a combo of soils and vegetation. Good rule is to use locking plastic levelers as they are designed for safety. Even then you have to use as directed and only stack as high as they recommend. Always ask if you can rent a level lot when going to Parks, and don't forget the sewage does not flow uphill if you're on a plumbed site.