You have a real problem here.  Mice urine is the worst.  I had this problem
with an oven in a rental!!  We took the oven apart, replaced all the
insullation and washed it down in and out and ended up throwing it out and
buying a new one.  Next we had stuffed furniture in storage and the
squirrels got in and not only tore some of it up, but deficated all over
everything.  I salvaged a couch and love seat by taking them into a
professional cleaning place I trusted.  That worked, but on a van athere are
so many crevices and nitches.  Anywhere the urine seeps into any kind of
absorbing fibers it will stay - and keep smelling - but I don't know if that
goes on for some time then eventually stops.  Houses have this problem when
rats get into the walls and make nests.   I recommend you talk to a US
agricultural service extension office and a mortician.  The mortician idea
sounds wierd I know, but morticians have some products they use to rid
smell.  A friend of mine got some spray one time that did the trick (but not
on mice).  Maybe that new product Fabrize is the off shoot of a mortician!!
Just a couple of ideas.  Good luck.

Madeline
15 Years of Westie Adventures
'90 Syncro Westie