I built a support system that bolts to the cargo uprights. Most rack connectors screw to the horizontal "surround" that connects the uprights together. Note that surround is heavy gauge sheet metal, not unlike the uprights. I went a little overboard and made my uprights out of solid aluminum. I did have to temporarily remove the luan interior to through bolt it. I did not want sheet metal,screws, even #12s. Indid put oak plywwod back in and varnished it. Suits us better than luan.
I have carried kayaks on Yakima racks, a cargo box filled with stuff. Rack includes crossmembers.
What appears as a gutter in your picture is most likely a trim piece to close the gap between the side material and the roof material, silver colored, like you see in my pictures. it kind of got in my way, but I worked around it. My cross members do stick out. I was not sure what to do, so I left the long material. Those horizontal extensions have come in handy for a solar shower, mounts for hi-lift, shovel, and other stuff. I would put on a center set for a RTT, maybe two additional. My trailer is 5X8. I thought the axle was too far back, but at speed, it does not fishtail.
Wife and expedition dog sleep inside, i sleep in a tent-cot.