Probably more than you want, but grounding is really about two things. One: amperage - how much current is the ground going to carry? This is a loaded question- you have steady state (normal conditions) ; transient state (turning something on or off); and abnormal conditions (lightning, fault conditions (think hot wire touches chasis); static, ect). Two: frequency of the current. The higher the frequency of the current in the three states mentioned above, the more the current will travel on the conducting surface. In the old days, really wasn't much of a consideration, because everything was DC , and only die hards would look at things like static discharge, and stray radio crap. However, with more and more computers, leds, ect. The more important proper grounding becomes. In fact grounding is a baine plaguing engineers for years. So simple in concept, but so difficult to achieve. Isolation, multipoint, single point, which to choose? Luckily for you, simple boat trailers have yet to advance to the point that most other devices have. If you decide you want to ground your trailer chassis in multiple points, I might suggest looking at where your overcurrent device is (typically those pesky fuse things) . You want to ensure all intended current passes through that pesky protection thingy. Adding multiple grounds can change current paths, especially under fault and transient conditions. The other major concern is darn corrosion stuff. For every connection to steel or aluminum you make with copper, or say brass. You have created the potential for a small battery (corrosion cell). It typically gets worse when you put it in water, and usually a major concern if you put it in saltwater. My recommendation is to stick with what the manufacturer, or what your wiring kit instructions say, clean the grounding point, apply some dielectric grease at the point of connection, and coat the exterior of the connection. Let me know if you want to get into the more technical details.