here in sweden some that have sprinters ( or other brands) has the pipe rack on 1 of the side

Why not put the pipe rack inside? No worries of the end cap coming off, no corrosion either. I keep all my PVC conduit on a shelf under my bench in my trailer, i also have a space for ground rods and another slot for wire mold. I know a trailer is different than a van but you could still utilize the space in a similar fashion.

I'll agree with putting the conduit tube inside. I had mine inside my Ford extended cargo and loved it. The sprinter should be long enough to accommodate 10' lengths. Another thing I have seen done if you have the hitop, is make a false floor and put pipe under it. I know there is a pic somewhere, I will find it.

http://i270.photobucket.com/albums/jj106/Romexking/electrical images/insidetruck3.jpg