This is where a little education and common sense can help.
You basically need three things: 1) Ohm’s law 2) a DC wire size chart 3) knowledge of your electrical loads.
You’ll use Ohm’s law to turn watts into amps. This is so you’ll be using the same units (amps) for all of your calculations. Always use “input” volts – where it’s getting its energy from, in this case 12 volts. Watts/volts=amps, for example, your 1000 watt inverter divided by 12 volts equals 83 amps. With inverters you should always add 50% due to inverter inefficiencies, and to handle surges, so 125 amps. Since 125 amps is too much load to run through your 40-amp charge controller, it has to be connected directly to the battery with a fuse larger than 125 amps. I would use a 150 amp fuse, and the fuse has to be within a few inches of the battery, so there’s less chance of a fire. Protect the positive battery connection with electrical insulation to prevent a short, and the battery MUST be secured to prevent it from becoming a projectile during an accident, and to prevent the positive terminal from contacting anything metal inside the van – even in a rollover accident. There MUST be a grounding strap between the grounding post on the inverter and the body/frame of the van, and between the negative terminal of the battery and the body/frame of the van. It’s also a very good idea to plug a GFCI in between the inverter and anything that’s plugged into it. A 1,000 watt inverter can electrocute you!
I typically use wire that’s rated for double the load that it is connected to. For each electrical circuit use a fuse that’s 20 to 50% higher amperage than the loads. NEVER install a fuse larger than the wire rating! The fuse must always be smaller than the wire rating, and slightly larger than the load. For example, if your DC fridge uses 6 amps and it’s within 6 ft of the fuse block, use 14 gauge wire and a 10 amp fuse.
Smaller similar loads can be connected to the same fuse, and connected in parallel, positive to positive and negative to negative. Larger loads should always have their own fuse - not shared with others.
Most charge controllers today are equipped to manage the load (some however, may be damaged by inverters, so that’s another reason to connect inverters directly to the battery). This is to prevent over-discharge of the batteries, as the controller will disconnect the load if the voltage goes too low – protects both the battery and some devices. So connect your 12V fuse block to the inverter’s “load” connections.
Be sure that your charge controller is designed for the type of battery you will be using. Yours is lithium (I’m assuming LiFePO4 chemistry), so make sure the controller and battery are compatible. Also be sure that your battery has an internal or external battery management system (BMS). Most good quality lithium batteries nowadays have a BMS built-in.
Like the inverter, the connection between the battery and the controller should have a fuse, and that fuse should be located very close to the battery. In your case with the 40 amp controller, install a 45 or 50 amp fuse. This set of wires will go to the battery connectors on the controller. Remember to use the appropriate size wire.
For the fuse block, and to size the charge controller, add up all your loads: 6 amps (fridge), 10 amps (two USB ports), 5 amps (LED lights), etc…. The sum of all your loads should be smaller than the load limit of the charge controller. The fuse block is connected to the controller’s “load” connectors.
The solar panels should be connected to the solar connectors on the controller. Be cognizant of the controller’s maximum input voltage. If you have a PWM controller your panels need to be connected in parallel to maximize efficiency, keeping input voltage as close as possible to 12 volts. If you have an MPPT controller you may be able to connect the panels in series to increase the voltage (check the owner’s manual). Connecting in series adds the voltage together in the same way as two 1.5 volt batteries in series makes 3 volts. Read the manual for your controller. Ground the panel frames or negative terminal to the van body.
All connections must be solid, tight, and free of looseness and corrosion. Only use STRANDED copper wire for anything in a vehicle. Solid wire will eventually vibrate and break. Again, use the correct size wire for the load, and don’t compromise on the connections/connectors. Protect all of the wire harnesses from chafing and abrasion, especially where they might move. Pull hard on all the connections to be sure they’re tight. If they come loose with a firm tug you’re not doing it right.
If your electrical panel area looks like a rat’s nest you’re not doing it right. Everything has to be tidy, tight, secured from movement, and protected from any kind of damage, heat, chafing, or corrosion. Protect from moisture. Even stranded wires can break if subjected to a lot of movement or vibration; better to secure and protect them.