I am having a hard time on which battery to use for my Aux battery set up. The second battery will be used to power a frig,external lights, charging camera equipment, sound amplifier while camping for about 3 night. What are you guys using?

Once I get around to it it will be an Interstate Batteries MTZ-34R. I have one already in my truck. I have a small connection to Interstate and they make a good battery.

I started with a regular starter battery, then an AGM/deep cycle like most people, but now I am making something custom and compact but with more Ah capacity using lithium ion batteries that will be built into the rear drawer system… still work in progress.

One of the main reasons was because I wanted to free up space under the hood, and the 2nd was because I wanted more performance and capacity.

no one can recommend a second battery without knowing how you plan to charge and connect the battery

Sounds like you need a deep cycle for your use. I recommend Odyssey or Northstar for their AGM deep cycle batteries. A wet lead acid starter battery after deep cycle discharges will be ruined.

Odyssey Group 34 front passenger under hood with C4 Fab tray

if you are using some kind of DC-DC charger, if it supports gel batteries they have a better cycle life than agm batteries. they are good for low amperage uses like you described, but if you just plan on charging in parallel with your starting battery it won't work. that's why i was saying we need to know how you want to charge and connect the second battery.

I agree it helps to know which direction you want to go, and also what your needs are. If you can figure that out first, and know what route you want to go, picking the battery becomes easy.

DC-DC provides more isolation and can charge and maintain the aux battery completely independent of the starter. Alternator and starter system voltage is completely isolated from this system on the AUX side.

The typical combiner, automatic charging relay, and similar device will combine or disconnect the two batteries in parallel. Generally, they are capable of moving a lot more current than a DC-DC. Sized correctly, could even be used to combine the aux for large loads like winching. Alternator is also in parallel when combined. Both sides share the same system voltage when combined.

Both system have pros and cons.

I generally use a DC-DC when working with custom voltages, like a 24v system in a 12v truck, or when needing to charge physically smaller SLA batteries, or better maintain AGM’s if the native voltage of the truck doesn’t fully support the top end charge voltage.

Here is a thread I made a while back that might help your research.
Blue Sea ACR - Automatic Charging Relays

So are dual kits from Off-Grid Engineering not preferable? I plan to use a compressor, light bar, side camp lights, reverse lights, and a fridge/freezer. I had planned to run the OGE dual battery setup with the NorthStar AGM's.

Build your own kit. You will learn a ton and most likely end up with a better result.

This...

Most of the DIY systems I see turn out even better than anything you can buy pre-made. You learn a lot, and can design and build to your own requirements.

I don’t want to name any brands, but I think some of the stuff out there is a complete rip-off.

Aside from the really nice but over-engineered battery trays, everything else is off-the shelf components you can get yourself. Yes the trays have bling factor, but whether or not that is worth it is a personal question.

You also don't need the ML series ACR unless you are winching. The smaller 7610 or 7611 ACR in my review is generally more than sufficient for what your needs are. Those units can be had for less than $100. Battery terminals, crimp lugs, and USA made welding cable is all inexpensive and very available.

group 31 odyssey in a removable arkpak battery box.

I also have a ctek dual battery controller and shrockworks battery kit that has being waiting to be installed for the last year, but I got hella lazy mainly because the arkpak is sufficient for my needs.....maybe ill drag my butt off and finish that installation.

I was planning to use the truck's existing charger and connect a solinoid in series with the second battery. The solinoid will be in the on position while the car is on and in the off position when the car is not running.

There's nothing wrong with it, but it will cost a lot more than doing it yourself. For the price of their kit and Northstar batteries you could probably make a lithium setup.