<< So, my question is... Can I install a roof mount antenna to the remaining portion of the steel roof, using the rest of the van's body for a ground plane without being greatly affected by the fiberglass roof? >>
Yes! You get about the same effect as people mounting antenns on trunk lids. The only real ground plane you will have is what is directly under the antenna as a flat surface. The rest of the body will have little, if any, effect on the antenna.
As for antennas - the Larsen NMO240 or 240B mounted using a properly installed NMO mount.
Go to www.k0bg.com
and look over the information there. Alan's web site is a gold mine of mobile installation information.