On the lumber forums, the general theme is to carry 4x8s with the tailgate down and resting on either 2x4s on the bed (so the whole board is supported) or on a frame that lays them above the wheel wells (obviously not a RamBox bed issue). Apparently, it is common for the sides to have notches before and after the wheel well to lay 2x6 boards sideways, creating a platform above the wheel wells.
When I was younger all pickups had 8' beds and the ads all talked about carrying 4x8 boards. Now that space has gone to the cab.
It used to be the case that a Suburban or equivalent, when the rear seat was down, could fit 4x8s. The backs of the second row were the same as the bed floor. And everything was locked up and protected from the weather. With the load extending past a down tailgate, it needs to be unloaded right away and is exposed to the elements, requiring moving blankets and/or a tarp for protection.
But now the SUVs have all turned into cars for transporting kids. At one point GM had a 4-wheel steering 2500 Suburban advertised for it maneuverability with trailers. They recognized that maneuvering around buildings, fences, and obstacles at the destination was as important as cruising on the highway. I came within a whisker of buying one. Now, no 2500 let alone 4-wheel steering. The biggest SUV is barely rated for a 9000 trailer.
The SUV's liftgate is also better for loading, especially with a trailer. Even the multifunction tailgate really will have limited access with a trailer attached and requires room in the rear to access it even without a trailer. Just blowing steam as I pine for a RAM 1500 SUV, where a 5-7 bed length would fit 4x8s with the second row folded.