sounds to me that something is jamming inside the starter (since it is making
the lights dim like there is a load on the battery but nothing is happening)
or the motor is locked up somehow.  Try to rotate the motor first by hand and
make sure that it will spin freely.  This may unjam your starter gear (if that
has somehow become stuck against the flywheel).  If the motor will turn freely
then check your electrical connections for security of attachment, and
corrosion.  If they all check out good then it is time to remove the starter
and test it out of the van.
Let me know when you get to this stage and I will help you with more advice.
Ken Wilford
Van-Again