12V Fridge wired direct to battery on the truck?

Dead wrong if you are talking about compressor fridges. If you are talking about 'coolers', as in a thermoelectric cooler like those POS things they sell at truck stops, then yeah, huge difference. 12v compressor fridges which is what most of use are running nowadays are the exact same as a 120v fridge, just a different power source. Lots of them are both power input types.

I've owned and istalled 20 or more 12v fridges by now, several in semis, several in campers and other vehicles. I have a lot of cars that my people travel in and they all have a 12v compressor fridge. Good quality ones last forever. The cheapest crap on Amazon doesn't.

If you are wiring a compressor fridge from battery run 10 gauge wire and a 20A fuse to it with half decent hardware and you'll never have any trouble, period.

 

I don’t run a fridge in my truck but if I did I would swing into a camper dealer and ask them what a good set up is?

 

A camper dealer is going to try to sell you the most expensive Dometic at stealer price or convince you that you some kind of old-school 3 way propane/DC/AC crap. Camper dealers are always 30 years behind in tech. It's what old people buy and by and large old people buy most RVs, but it's changing.

The info is all here, and everywhere online.

Truckfridge (.com)
Dometic
ARB
Vitrifrigo
Engel
Novakool
IndelB
National Luna

 

12V is really the only sensible option for a truck. Anything running off the inverter is wasting 15-20% of their power on conversion losses. That's why I'm trying to convert as much 120v stuff to battery through converters so there's less loss. Already made a 12v plug-in for my monitor and just waiting to figure out how I want to hook up my mini pc power supply to the batteries. 24A ain't no joke.

 

As with the first problem it's almost always an issue if contact resistance. Big lugs should be closest to the battery. Any additional lugs or terminals under it increase the resistance a lot. Same with keeping things clean. Degrease and if it's looking crusty a bit of scotch brite should help. Ideally you should hook these accessories up to a bus bar inboard so you can have the minimum number of cables hooking directly to the batteries.

 

Forget the scotch brite, get the wire brush. That's bad. If you have to you should replace the parts. My company uses a wax kind of stuff to coat the terminals and protect from corrosion.
https://www.amazon.com/dp/B0BRBSD2BS
Dielectric Grease on Battery Terminals: What Is It and How to Use It?

 

The bus bar is really just if you have multiple high draw items. The important thing is not to stack more than a few on the batteries and to make sure the high draw main tap is first in line and clean. The more crap that accumulates the lower the voltage goes when drawing high current and also leads to heat. Clean terminals and dielectric grease are important to make sure it stays good for a long time.

The nuts/studs are only really there to clamp things together. So ensure cable contact points are the most clean.