Allen,
Many threads here about insulation, as well as over at the Sprinter Forums(Sprinter Source). I've done a ton of reading on the subject, and here are some conclusions I've formed:
-Stay away from anything that holds water, like fiberglass, cotton, jute, etc(especially if you live in humid/wet environments)
-Dynamat is going to cost a ton
-Reflectix seems to work well, though many question it's actual R-value
-If you have the $$, check out some of the spray on insulations, like Lizard Skin as the "first coat" in insulation. You can then Dynamat/Second Skin/Reflectix over it.
-Check out Second Skin Audio for more products(again, $$)
-Also check out
http://www.enguardinsulation.com/ This is what I plan on using in my build, along with second skin/dynamat in certain areas
Typically, you see many builders insulating, wiring, then sheeting the sides and ceiling of the van.
Hope that helps!