Worse yet, it's under the "permanent" floors so whatever you do won't be very easy to see or modify in the future. I started out wrapping a sheet of uhmw polyethelyne around the step tube and securing with safety wire. Probably would have worked OK, but it bothered me that it would be impossible to replace if it wore through. So I built a little bracket out of scrap Al and installed a fairlead made for the exposed cables on the lift struts of old Pipers (J-3, Supercub, etc.). These wear for many, many years on these old airplanes - and the cables are under considerable tension, unlike those in our RV's. The bracket is pop-riveted to the step tube. Wish I had a picture, but, alas, I don't.
I've seen pictures of where a depression was created in the step tube to accomodate the cable - either by cutting out a divet and welding a curved patch or simply by smashing the tube a bit with a heavy pipe. Neither appealed to me: the former seemed like too much work, the latter too crude.
Hope this helps. E-mail me if not clear and I'll try to clarify somehow.
John Miller