Can't say about strength of brackets other than what was said above. I would also worry about total height for going under low bridges and wires strung across the road, where there are government rules about all that.
There was an interesting thread a while ago about how to measure your high point from the ground. It was trickier than you would expect to get that right. Complicated really.
Some people paint over the shower/tub skylight to cut down on the sunlight/heat. I drape a towel over it held down with bricks, but that only works while parked, not driving down the road. (Our living room skylight has its own shade fitted)
