As mentioned weight and friction do most of the work in my opinion. The tie downs are to keep the movement from ever starting. Most TCers use some sort of rubber mat between the TC bottom and the truck bed to add a bit more non-slip to the rig.
One other type of system for tie downs that also replaces having to have a jack system is the
www.stablelift.com Several of the forum members have such a set up and speak highly of it. It actually clamps the TC to the truck bed.
For the most part, on a trip when the camper settles in and gets squared up to the front of the bed or whatever you have it up against, a spacer, etc normally there is very little movement on the rest of the trip. This past summer I had to make some small adjustments on my tie downs about half a day into the trip and for the next two months they didn't have to be touched. Many times you can catch a sale on any of the major brands from many dealers, or find them on some free list such as Craigs. Some of the parts are truck specific, so you may have to look a bit to find a good used set as they seem to last forever. LOL
