I had the same question a few months ago and after sifting through a lot of info. decided to add a second battery with an isolator for my 2021 P1K5 LE.
I have a winch and light bar and use both for plowing quite a bit, which means lights, shifting and lifting. I also ride way, way out there alone and like the peace of mind of a second battery.
There is a key difference in set ups that others noted above. You can parallel two batteries, hook new accessories to the main battery, and that will work just fine. The batteries will, however, live together and die together because the lower-charge/failing battery will suck down the stronger battery.
If you use an isolator, put all of the new accessories (esp. high draw items like a winch and light bar) on the second battery. That will save the main battery for starting and shifting, while putting the add-on load entirely on the second battery. If the second battery is low, you can still start and drive the machine and it will re-charge the second battery or at least get you home. And if your main battery dies, you can use the second battery to jumpstart.
Gratuitous thoughts: the winch should be tied straight to the battery because of the high load, but I really suggest using an automotive relay and car/marine fuse power block for the other accessories - about $20 for a relay and six tap fuse block. Using a fuse block makes adding new accessories super easy and you can get rid of the in-line fuses. Rather than hunting down scattered in-line fuses, they will all be in one place - just make sure to use the right amp fuse for the specific accessory.
This is my dual battery set up. The isolator is just out of the picture (mounted in the flat oval near the anti-freeze fill cap) and the winch relay and breaker are mounted out of view on the right. This shows off my super-engineered Gorilla tape battery lifter (we've come a long way from duct tape, baby!) and I also added a 30A circuit breaker (far left) primarily so I can cut the power to the system (about $20 for the circuit breaker). Don't tell my wife, but I didn't really need the circuit breaker, I just thought it'd be cool.
Negative is hard-wired from the battery to the negative bus on the new fuse box (out of view on bottom of the fuse box).
Positive runs from battery to top of the 30A circuit breaker to the relay to the new fuse box. The relay (next to the fuse box) is a standard 30A automotive relay that is key on power (KOP) switched from a PosiTap on the factory acc. fuse. You need to pick the right side of the acc. fuse to tap, so read about it; I forgot which one it is and don't know if it's different in other model years.
All of my accessories and their relay switches run through the new fuse box (winch relay, light relay, dual battery voltage meter, wireless winch remote).
Good luck!