Filling inside rear tires

Coming back to visit this one with an update. After some roaming around trying to get answers, I was referred to Borg Equipment (shows up on the interweb as http://yourtireshopsupply.com/), where I was able to get long valve stems bent specifically to fit my year of Sprinter. I had them installed at a local tire shop, and they look like the solution. They are sold with rubber stabilizer fillers for the vent hole in the outer rim through which they exit, and they're right simple to reach. It was pointed out to me that they'll make rotating the tires more expensive, but I rarely rotate tires anyway, so I'm not worried. I got great service from Kelli there. We haven't taken the rig on the road yet, but I'm impressed so far.