I cut card board the size of the panels I wanted and found that I could not provide enough room to get around on my roof. I tried placing them all around.
I decided I wanted long/narrow panels so I found some that was 23"X 57" 100 watt.
I again did the cardboard thing and found the perfect placement and accomplished the ability to get around on the roof along with enough room to replace the 12X12 vent over the bed area with an escape hatch.
link to my install:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/24783458/srt/pa/pging/1/page/2.cfmJim