I have the same issue with the rear duels being to close.My truck is a 2003 gmc,it must be one of the few 3500 which came with good year tires with a D rating.One dealer said it was because it was two wheel drive.With just the camper on the truck I don't think they touch.My last trip I also towed a 20' enclosed trailer and I'm sure they touch on occasion.I posted a thread on spacers a while back and was steered to freds as being one of the best.I have not went that route yet as I was going to replace the tires with E rated in hopes the side walls would be stiffer.After reading this thread I'm having my doubts.
