In your picture you have several lines labelled with a wire size and fuse rating. My inference is that the actual physical location of the fuse is not specified.
The rule for fuses is simple. The point is to not burn down your house from a wire that is too hot from too much current. The rule is every wire must be protected from too much current at every point where power can be applied. At any point along the length of the wire it can chafe against a sharp edge and form a short circuit.
For example, you have a wire from the solar charge controller to the battery. It is AWG #6. According to Wikipedia that size is good for 55 to 75 amps depending on insulation temperature rating. The two solar panels can only generate at most 6 amps each. The wire is protected from too much current from the solar panels because of the current limited nature of the source. A fuse isn't necessary. If you used tiny AWG #22 wire you could protect it with a 3 amp fuse. The wire would be safe from igniting but not useful as the fuse would blow shortly after the sun came out. The wire has to be big enough to carry the maximum possible current to be useful. In this case that means a fuse isn't necessary.
At the other end that wire connects to the battery. Even if the battery only has a 500 cold cranking amp rating it can deliver 1000 amps into a short circuit. A battery has stored energy. A solar panel doesn't. At the battery end where the wire connects to the battery it needs a 55 amp or less fuse. The wire from the battery to the fuse needs to be short as possible as it is unprotected.
Normally that wire carries energy from the controller to the battery. It only needs a fuse to protect it from the battery. If all the wire to the charge controller and the solar panels is AWG #10 or greater then you can put a 30 amp fuse at the battery and all the wires all the way out to the panels are protected from the battery.
The solar charge controller in your picture, Rover 40 amp, is the one you want. It could happen that it fails and you get a temporary replacement like the one I have. That's the $12 PWM from amazon, the blue one. My controller has the load plus terminal tied to the battery plus terminal tied to the solar plus terminal. The magic switching is done on the load minus and solar panel minus terminals. If my solar panel plus wire chafes against the roof rack and shorts out, my fuse at the battery end of the charge controller wire will be protective. Even if your controller contains electronics that will turn off or fry like a fuse, a temporary replacement controller might not. This is common.
With a 30 amp fuse at the battery you don't need a fuse where the solar panels connect to the solar charge controller. With more than 600 watts you could have more than 30 amps of charging. At that point the charge controller would be fused for more than 30 amps. That means that the solar panel wiring could chafe and short and be exposed to more than 30 amps. According to wikipedia, AWG #10 wire is good for 333 amps for 10 seconds. That's long enough to blow a 40 or 50 amp fuse at the battery so you still don't need a fuse where the solar panels connect to the solar charge controller.
Similarly, the wire to the fuse block needs to be fused at the battery with a 55 to 75 amp fuse. 80 is good enough.
Your max current is 80 amps. The wire to the fuse block is long so the size is big so that you minimize the voltage drop. The 4 inch piece of wire making the series connection between the batteries certainly can be 2/0 huge. If you use the much easier to work with and cheaper AWG #6 like the rest of the system the voltage drop over 4 inches will be insignificant.
If you later add a wire to charge from the alternator the starter battery can fry the wire from one end and the house battery from the other. You need a fuse at both ends.
My system is odd as I have a starter battery that is upgraded to a deep cycle battery. I have a 9 foot wire from the 80 amp fuse at the battery to the fuse box behind the driver's seat. The fuse box has a 30 amp fuse to protect the wire to the solar charge controller. My solar panel AWG #10 wire is also protected by that same 30 amp fuse. All the loads connected to the switched load output of the charge controller are also protected by the same 30 amp fuse.
Amazon lists the Rover 40 amp MPPT controller for $164 and the Wanderer 30 amp PWM for $30. If you have room on the roof of your van you could put up a third 100 watt panel and use the PWM controller. Starting with 200 watts and PWM, the MPPT upgrade will give a 10% to 20% increase in harvested energy. A third panel will give a 50% increase for less additional cost.
