I'm not certain about the icom 2820..I've never HEARD of one having an 11 meter conversion done on it, but I know the yaesu 857d is a very good sounding rig on the 11 meter band, even on AM (not just SSB).
Fiberglass roof makes roof mounting antennas a moot point, no antenna worth noting would get out well when mounted to fiberglass like that. Best option is going to be GOOD ball mounts (an old Antenna Specialists ball mount, NOT a radio shack piece of trash) bolted to the side (need access to the space between the outer sheetmetal and the inner sheetmetal for this), second best would be a couple of good heavy mirror mount brackets, screwed into the side metal with ATLEAST 4 #14 stainless sheetmetal screws, but bolting it with four 1/4" bolts would be better if you can get to that same space as described for the ball mount. You can get the mirror mount brackets at radio shack, but buy a higher quality stud mount for the antenna, such as a predator 10k hex mount. You can buy predator 10k hex mounts at
www.predator10k.net.
As far as the antenna goes, a wilson 5000 trucker would do well, just make sure to use a long enough bottom shaft to get the coil above the roofline..it doesn't have to be far, just so long as the entire coil is above the top of the roof.
For the scanner, I think a through-glass antenna works OK for them..just stay away from the through-glass antennas for the CB and ham rigs.
If you operate in the 2 meter band (or any band outside 10 meters really) you're going to need atleast one more antenna..I've only used home brew antennas for 2 meter, so I can't offer much advice as to brand, but figure on a fender lip mount, or another mirror mount on the OTHER side..If you can post some pics of each side of your van, I'll see if I can come up with some ideas for specific spots for mounts.