Boggles the mind..A full size Port-a-Potty inside your van...

Thetford makes several "hassock" style portable toilets.  Camping World
sells em (search for their site on the web)

The best IMNSHO is the smaller thetford that will slide between the front
seats.

Sears also sells these "thunder boxes".  Theirs are made by...Thetford of
course.

These are all high tech solutions.   Phaedra mentioned a lower tech but
certainly suitable one that uses a seat on a plastic bucket.  I disremember
the maker though.

Cheers,

pensioner