Reddit mentions of 120 Pieces - EPAuto Assorted Car Truck Standard Blade Fuse Set (5/7.5/10 / 15/20 / 25/30 AMP)

Get a fuse box like this then attach wires to cigarette adapters. The fuse box is connected directly to battery with another fuse between them.

EDIT: Mind you, I'm still trying to figure out what size fuse, circuit breaker goes between them LOL

EDIT #2: Also, from this fuse box, you directly connect LED lighting, your cig adapters and any other 12v source you may have. All the fuses are in one place instead of in-line. Makes it easy to diagnose problems as you can just pull the fuse to work on something. Make sure to label!

EDIT #3: Also.. get some these fuses. It's cheap and now you have a ton of extras!

The vehicle has a wire from the alternator to the battery. It is usually too small. If you connect your house battery charging wire at the alternator rather than at the vehicle battery you will improve the charging of the house battery without adding additional current and voltage drop to the charging path for the vehicle battery.

The wire from the vehicle alternator to the house battery gets the switch somewhere between the two. It doesn't matter where. If it is near either end or in the middle it doesn't matter. The switch instructions say to put it near the vehicle battery. It doesn't matter. The ohms are the same no matter where you put it. Make the wire as short as possible. Locate the switch out of the weather and wherever it is best to have a short total wire length.

Fuses protect wires to avoid fires. The size of the wire sets the upper limit of fuse size. The wire needs to be protected at every source of power. Your house battery charging wire has a connection to two sources of power. The house battery and the vehicle battery / alternator both feed the wire and both need a fuse. Mount the fuses as close as possible to their power sources.

The inverter takes a large DC current. A 440 watt blender using a 90% efficient inverter will need 40 Amps running, maybe twice that starting. Mount the inverter as close as possible to the house battery. If the inverter has its own fuse and the cable from the battery to the inverter is so short that it can't touch any ground even if cut or disconnected then maybe you don't need a fuse between the battery and the inverter.

The fuse box I used is:
https://www.amazon.com/OLS-PSZACCEPS051H-6-Way-Illuminated-Blade/dp/B00QMTAZ1W/ref=sr_1_1?s=automotive&ie=UTF8&qid=1478725271&sr=1-1&keywords=automotive+fuse+block
You connect a wire from the house battery to it, put in the fuses then connect the fuse box outlet connectors to your loads. Again, put a fuse at the battery to protect the fat wire to your fuse box. If the wire to the fuse box is short enough so that even if the nut holding the wire on the fuse box falls off the wire can't reach anything grounded then maybe skip the fuse.

Specific answers

1 and 2) Wire from alternator - fuse - fat wire - separator - fat wire - fuse - house battery. Then from house battery to inverter, house battery to fuse box, both with fuses if necessary.

3) fuse size depends on wire. For small loads just use #12 wire and a 20 amp fuse. For your fat wire for charging the wire size depends on the length. If you mount the house batteries behind the driver's seat and the alternator is on that side of the engine you can probably get by with 10 to 15 feet. At that length #6 would be the smallest I would recommend. For #6 I would suggest 80 amp fuses.
https://www.amazon.com/gp/product/B00W5SG2C0/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

4) Standard blade fuses fit in the fuse box linked above, not mini.
https://www.amazon.com/120-Pieces-EPAuto-Assorted-Standard/dp/B01DYQ5T3O/ref=sr_1_5?ie=UTF8&qid=1478727709&sr=8-5&keywords=automotive+fuses

5) Grounds are equal in importance to the +12 wiring. Any resistance will drop voltage and cause problems. When 2 items share a ground connection and that connection gets high resistance the symptoms get strange and diagnosis gets difficult. For a battery ground you need a hole drilled and a bolt run through. Star washers on both sides of the steel are helpful. You must grind away any paint. Find some ground connections that already exist to see how Sprinter did it. The battery black wire is like what you need for your house battery. Small wires around tail lights are like what you need for your small loads. The inverter should be connected with a short fat wire directly to the battery. The separator switch needs a small ground like other loads. There may be small grounds around the van that you can add to.

6) The fuse box doesn't get a ground connection.

7) The biggest risk (but least likely) is burning down the van with an electrical fire. The most likely risk is making some small arcs and blowing some fuses. A likely risk is connecting something backwards. LEDS just don't work backwards. If you connect your inverter backwards it will likely blow internal fuses, maybe fry the thing. The fan may go backwards, maybe fry. If there is arcing that involves alternator current it could kill the alternator.

Now the fierce criticism. Those coolers will kill batteries and not keep your food cold. If your van is 90F inside and the cooler drops it 30 then inside the cooler it is 60F. Not cold enough. 55 Watts is 110 Amp hours per day. You need to have a volt meter to keep from killing your battery. Do not wire an inverter like you wire a phone charger. The inverter needs short fat wiring. Search the internet and you tube about crimp connectors for fat wire and for #12. Don't strip a wire and twist it around a bolt and then tighten a nut. Look at the way wires are done from the factory in your van. No sharp edges, no loose unsupported wires. Before working on vehicle wiring disconnect the battery minus wire.

For the sake of argument, I actually have the products I have linked. I have only one battery and no disconnect switch. I upgraded the vehicle battery size. I have a vent fan, LED lights, chargers, no blender, 100 Watt solar. That makes me informed and experienced, not expert.

Blue Sea Systems 5035 ST Blade 6 Independent Circuit Fuse Block with Cover https://www.amazon.com/dp/B006VELERM/ref=cm_sw_r_cp_apa_qzcJBbGV5KY2Q

120 Pieces - EPAuto Assorted Car Truck Standard Blade Fuse Set (5/7.5/10 / 15/20 / 25/30 AMP) https://www.amazon.com/dp/B01DYQ5T3O/ref=cm_sw_r_cp_apa_dDcJBb1W0PM5K