that is not excessive at all.. after all 1000 lbs is only the weight of 4 healthy people on the roof...
skoolie roofs have huge support structure, as they are not crushed when the things get in a wreck.. after all, they are built to protect the crumb crunchers...
the ONLY disadvantage i see is that the additional weight being like 12' off the road, which pushes the center of gravety higher... which means ya gotta be way more careful driving around corners, especially in wet weather...
i havent put solar panels on my roof yet, will do so this year sometime, and am not concerned about the weight at all.. i think i want somewhere around 1000 watts.
again, always remember the center of gravety if you are driving fast cause things are fine until in the middle of the corner the bus almost tips over.... been there both with freight loads with the peterbuilt and with my flx bus when being an idiot...