S. Heisley wrote::thinking: I think I would finish that flat roof part with something solid, maybe a couple 1x1' boards from side to side and a sheet of foam, glued fabric and paint. This is because a plastic sheet on a flat roof surface like that, with very little support, will quickly begin to sag. If rain comes, it will settle in the sagging area and make things even worse and the plastic will eventually leak or break. That's my opinion.
I agree with Sharon's suggestion. I tried a tarp with and without a plywood subsurface. Without the plywood, support was under the (8 foot long) center line and along the 4 edges. There are (5+ foot long) cross members about 32 inches apart, but they are about a half inch below the stretched tarp top. This works great when the roof is elevated (tipped), but water can form shallow pools when the top is in the horizontal (closed for travel) configuration. So on the rare times the unit is stored closed, I lower the tongue to slope the roof enough to spill water forward. The roof is horizontal when towing, but highway speed blows the water off.
My tops were well stretched and did not "quickly begin to sag", in fact they never did. I tried tops with and without anchoring along the center line. Tops without the center anchor would display some rise during tow at very fast highway speed, but even after hours of such 'stretching', the top did not sag when we reached our destination. When the center was anchored along the center it was rock solid on and off the highway. They 'bounce to the touch', but standing water is never a good idea for any roof.
UV damage is a concern. Over one winter (Oct-May) I used a cheap blue tarp as a temporary roof, and kept the top raised pointing due south to gather as much sun as possible. To no surprise, the UV took its toll on the tarp seams. I replaced it with a (10 mil) reflective tarp which lasted from May 2015 thru July 2017. I have been told that painting the tarp would prolong its life.