We have 4 cameras, (expandable to 8). The entire system is 12v, kit has 110v plug ins for cameras and monitor. Instead of cutting off the leads from the adapters, we purchased new coaxial plugs with pigtails, which we spliced into the existing feed wires. Old system used hardwired feed to monitor, so we just omitted the data wire. That allowed us to reuse the existing wiring runs already installed in the boat. ;The system was inexpensive, and pretty easy to install. The system also allows remote access, as long as you have a wifi signal to the boat.
I have no idea what the tech support might be like though, as we haven't had to use them. Time will tell. Hope this helps.
https://www.amazon.com/Expandable-Wireless-Security-Hiseeu-Surveillance/dp/B07719HQQS?th=1