http://en.wikipedia.org/wiki/Long-range_Wi-Fi
http://www.instructables.com/id/Uni-Directional-WIFI-Range-Extender/

As for an access point / repeater, I've set this up using 2  Linksys
WRT54GL routers at home.  You could do the same, and run the Linksys
from an inverter perhaps?

Here is what I did at home, basically, to get the 'internets' down to
the kids study/playroom, without having to string cat5
wiring the length of the house.

There are commercially available WDS / Repeater type of deals
out there as well, but cannot vouch for their effectiveness.

Why do you need multiple network connections in your van?

You can also setup wireless on a windows laptop to act as an
access point I believe.

Used to be able to do this on a Mac as well, but I've not tried it
as of late.

hope this helps

 -tim

90 Syncro Westy
Gilbert, AZ
http://www.timschneider.org