<<Does anyone have any experience with creating a "basement" storage unit on
their van...  >>

That sounds like a good idea - never seen a kit though - even in the Reimo
catalog which seems to have just about every accessory imaginable. I've
mused about a similar modification - to store jack, tools, emergency
supplies and other stuff you usually don't need - between the frame rails on
the passenger side. It also seems like it would be a great location for a
water tank (instead of the normal camper location) or a fuel tank. Mike
Sohnlein did in fact install an aux. fuel tank in that location - you can
see a picture of it from http://www.westfalia.org/  if you follow the
"Follow the link to post and view your favorite Westy pics" link to the
Yahoo photosite. (It's kind of a drag because you have to "join" in order to
view the pics.)

Re the speed bump issue, consider the following: 1) the standard vanagon
propane tank must be at least 9" high and mounts in the same location you
are describing, except on the driver's side 2) On the new eurovan campers,
the propane tank must be all of 5 inches off the pavement and 3) I've driven
a Honda CRX with about 7" of ground clearance for 150,000 miles and never
bottomed-out on a paved road.

Love to hear details of what you end up doing!

Doug Fitz-Randolph
Yarmouth, ME
'90 Syncro