As I understand it, the idea is to inhibit, or better still, completely eradicate fretting corrosion. When we were having our camper built in 2009, we intended to place it atop a steel tray. Being steel to steel, fretting would certainly occur, the slide-on camper manufacturer suggested we use a specially treated timber spacer to virtually eliminate fretting corrosion. We did, and it has been successful.
I took these pictures a few minutes ago of our camper, which is now atop an aluminium tray on our truck. As you can see, it runs the full length of the camper sliders. Quite some time ago we were camped alongside a similar camper arrangement that was steel to steel. We could easily detect fretting corrosion occurring, which while apparent to the owners of the camper, it wasn't something they thought there was anything they could do to stop the corrosion happening.
The last picture is the most important one for you. There you can see the flexible packing between the chassis rail and the steel tray frame built by our second stage truck manufacturer, which in this case was All Terrain Warriors. With Isuzu in Japan, being the first stage manufacturer.
Mick.
