question

You would use the load ports for any loads you wish to power with the MPPT and have some additional control over them. These ports can be turned on and off for various reasons and to protect the battery from over-discharge. They are always at battery voltage, and are current-limited. They can't be used for something like the input to an inverter.

Your BMV should be connected to the battery per the installation instructions.

Diagram is ok, but a couple of points.

Load output connection is doing nothing and should be discarded. Generally leave it disconnected unless there's a specific use.

If the BMV you're going to use can monitor the van battery (e.g. BMV 712), you can add that connection, fused at the van battery.

You might find it easier to install multiple fuse boxes local to their loads. Will save wire, time, but at the cost of slightly more time spent if a fuse goes. I ended up with 4 in my van. Size the wires appropriately and fuse the feeds to the fuse boxes.

A practical example: I use the load output of my MPPT to power the anchor light of my boat. Using the Streetlight function of the MPPT controller it is turned on shortly before sunset and off about half an hour after sunrise. The MPPT uses the voltage of the solar panels to detect sunrise and sunset. I set them to 12.95 V for sunset, as that turned out to be a good match, and 13.45 V for sunrise as that value must be 0.5 V higher than the sunset threshold. The light is then turned off with a 30 minutes delay.