Your plan avoids my complicated installation, but adds the job of dragging and connecting the "big" wire each time. It does not have more power, unless you obtain a model bigger and even more expensive than the $220 "360 watt" version. It would almost certainly support more than "360 watts", because you will have more than 13V on the input side while the Truck is running. The 30A limit (which I SWAG to be the actual operating limit) probably allows for a bit more than 400 watts on the input side, with only a small loss in the conversion.
But my design, through the Bargman cable, is allowed to pull about 450W into the MPPT, and the MPPT Voltage conversion cost is also a mere 3-6 percent. My implementation avoids the extra cables, generates MORE power, and costs much less in purchased hardware. Instead, it costs time and effort installing the parts - just once.
Your method probably calls for a 2-conductor cable, rather than just a single 8-AWG wire. While my method pulls only 12-13A along the Bargman path, yours will pull 30A -- in both directions. Within a Bargman cable, the return ground wire is always built bigger than the TBC wire, but it might not be "good" for a full 30A continuous load (I usually see #12-AWG, and even #10-AWG would be at its design limit). The total thermal insulation value of the cable is very high: Ground wire insulation, PLUS the insulation on the surrounding wires, PLUS the outermost foam wrapping and cord cover. When these cables fail, it is often "voltage drop" power loss (heat) creating high temps within the conductor, travelling all the way down the cable to the plastic end assembly, and melting the end assembly. The wires don't often fail in the middle - when overloaded, they fail by melting the 7-pin plug assembly.
Your design is simple, and otherwise sounds great. We share the use of a separate "big wire" battery connection, with its own fuse. (Mine is 50A, although yours can be just 40A, because less power is being provided).