As Almot mentioned, perfectly flat will cause issues with moisture. Rain, morning dew, the morning frost as it melts off etc. Moisture will cause puddles on your panels that will have considerably more of a negative impact on power production than I think most people expect. I've currently got six 100 watt panels on my 5er roof. I made my own brackets out of two pieces of L aluminum stock bolted together for each bracket. The back of the trailer where some of the panels are mounted has both the rounded curve, and a downward slope towards the rear of the trailer. I made the bracket length so that the rearward slope is removed, but kept the curved roof slope for water runoff.
I have 3 panels on both sides of the roof so half are generally somewhat more optimized towards the sun than the other three. It's a compromise, but anything other than auto tracking panels are a compromise, and tracking units aren't really a viable option on an RV.
2015 Ram 3500 Laramie CTD, 4X4, AISIN, B&W Companion Puck Mount
2016 Heartland Bighorn 3270RS, 1kw solar with Trimetric and dual SC2030, 600 watt and 2k inverters.