I also run an ARB 50. Used it a few time for a few days in Moab this year. Worth its weight in gold. a few things.
The Cig lighter adapter..... Mine was hit or miss. There is a decent voltage drop when running thru cig lighter adapter. Sometimes the compressor would kick on and it would fault with the "no power light indication, fault light flash @1hz" looking in the manual, it said it was the "low voltage threshold reached" I measured the voltage and it was above the med cut off, that's when I had to make a test harness and measure voltage when the compressor kicked on, I seen that when the load was applied the voltage lagged and did drop below the threshold. After looking at it, I found it I turned the cig adapter approx. 45 degrees, I was able to decrease the amount of voltage loss by almost 0.7V.
Of course I had already had my fridge in the truck for a week prior to my trip to Moab, so I was worried since I found this issue the day I was leaving.... Just my luck. After doing this I was able to make it thru the trip with no issues, 5 days, fridge in truck the entire time while I was out rafting. Every night, everything was nice and cold and it was WAY better than a dam cooler. The guys on the trip with me asked me to hold some stuff for them as I emptied it thru meals.
I hardwiring a 10G wire from the battery fed blue sea fuse box, to pass rear wheel area (about where the seat back retention bracket is) then using a suitable size Anderson power pole set will make a QD there for fridge, and modify the 12VDC power cable accordingly. Overall I am pleased with the fridge and its performance, but I was a little worried about this issue the day of the dam trip. I have a engineering background and had the test equipment (DVM, current clamp, Sorenson 33A power supply, and a box of cables/ adapters to rig up a test cable, etc) to debug what was going on, otherwise I think I would have been screwed. Also found that one of the 2 ground springs on the ARB cig adapter was open, so everything relied on the one spring/ loop deal to provide return contact.
The 50QT (or same footprint shorter, 37qt version) fits like a glove on the 40 side fold down in the X. I used the ARB hold down brackets but the straps were to short so I bought some 6ft tie down straps from Raingler nets that fit the ARB brackets perfect and just put the fridge on that. The tiedowns are going to the ARB brackets where the seat is tied into the body of the truck under the rear seat. The 2 holes on the front bracket were perfectly sized to accommodate the hardware and positioning. The rear brackets I had to drill 2 holes in the seat bracket, but it was not hard. Ill take photos of this later. All in all, its a great setup.
Here is what I have uploaded already, but you can see the fridge on the 40 folded down. the seat bottom can be folded forward like this so you have more room when doing shuttle service on the river OR remove it like I did and use that area to hold more cargo (quick access snacks, water bottles, my kit bag, and stuff you get to while driving.
And incase you are wondering what 50QT will hold....
On my week long trip to Moab this summer. Started with a half gallon of moo juice, 52oz bottle of boathouse farms green goodness, 59oz bottle of simply orange juice, 8 chobani yougurts, 3 packs of sliced deli cheese, 2 packs of sliced deli meats, 5 pks of sausages, a dozen eggs, a small nalgeen filled with real maple syrup (is there anything better than that), 5 apples, a Quart ziplock filled with chocolate bars, a 1gallon ziplock filled with chopped onions and peppers for the dutch oven meal I was to prepare (hate chopping in the field if I can prevent it), a frozen bag each of meatballs, steam veggies, and steam seasoned potatoes wedges (tinfoil dinner cheating, since already cooked it heats up fast on the fire and the taters are actually cooked when everything else is hot), a 6 pack of sprite and coke, 8 frozen waterbottles, and a few little things here and there.
After the tin foil dinners I swung by city market and picked up 11lbs (yes eleven) of boneless pork country ribs for the Dutch oven dinner, and a half gallon of ice cream (place ice cream in the bottom rear most section, meat in the front on the bottom, pile everything else on top, and drop temp to 15*, it will keep the ice cream frozen and meat will not freeze after 24 hours (it will firm up but not freeze in my case).... Temps were 90s in the day. I did place a pillow on top of the fridge and rested it against the arm rest on the rear seat to prevent direct sunlight on the fridge, but be careful not to block vents in the bottom rear of the cabinet.
Honestly, after I have lived out of a backpack camping for 2 weeks straight before, I know I could go a month with this fridge solo if I had too and have gourmet meals. Even the girl said "this makes camping nicer, but I wish we were in a hotel...." she did not like tent camping in Moab while it was hot.
Fridge just sitting on seat getting an idea on mounting
ARB 12v harness modified with Anderson power poles
Anderson powerpole in Rig for power
Rigging to secure fridge to vehicle, rear is the same.
Fridge open showing accessible room, the lid can pop off if needed but I can get a gallon jug out with no issues.
And still have room for a few reusable grocery bags between the fridge and front seat that the rear passenger can still access.