I did the platform bed thing once too and it worked out really well, you'll really like the convenience. Mine was of 2x2's with a hinged plywood top wrapped in that automotive fabric that had 1/4" foam adhered to it. not supper comfy but OK in a pinch. I also used some old worn out lift gate gas struts to push open and hold the bed/lid up. It worked like a champ and almost too strong to close.
You'll need books on tape, cd, mp3, something that will really grab your attention. (we once drove 36 hours to DC and couldn't wait to get back in the van to hear what happened.) lol You will find that you have brought too much stuff. Do a short 3 day trip to see what you really need and adjust before the long trip. Avoid the water proof bag on top of the van as it kills your aerodynamics and won't be water proof. I found that an aluminum truck tool box bolted to a rear basket that fits into a receiver hitch works really well.
I look forward to your trip and updates.