Custom Design of a 12V Battery (Protect) Disconnect Solid State Relay

Most RVs come with a battery disconnect switch to turn off electrical power. In our Hymer Aktiv Class-B camper van, the battery disconnect relay (or switch) is a latching type that controls the DC power only. The inverter for AC power has its own on/off switch for independent control.
At the beginning of this video, a brief introduction was made on different types of relays. After that, a custom design of a solid state relay planned for replacement of the latching electromechanical relay was discussed. The relay is programmable for low voltage cutoff levels and on/off delays. Its circuit design and simple usage operations were demonstrated. While my application is for a system with 30A DC loads total, the design can be used for higher amperage using a corresponding circuit breaker.
The MOSFETs used are P-channel type for high side switching (positive line), each rated for 120A and a low drain to source resistance of 3.8 milliohms. Ultimately, the current carrying capacity of four MOSFETs in parallel will depend on the MOSFET to MOSFET variations and the size of the heat sink.

[@chipbrown433]

This was interesting to watch. I find it interesting that the circuit breaker is the higher power dissipation device. It's easy to get tunnel vision on the FET resistance when after a certain point it becomes far less significant.
After looking it up, I'm not surprised to see that you had a tough time sourcing the obsolete Infineon FETs. Surface mounted P-channel FET SIRA99DP-T1-GE3 is in stock at Mouser and IPB180P04P4L02ATMA1 will be very soon and both have a lower RdsON, but you do have to manage heat transfer through the PCB rather than directly on the TO-220. And of course you need a circuit board. It looks like you're set for your one-off implementation, but if you ever want a little PC board to hold some/all of this let me know and I can probably whip up some artwork for you to send to somebody like OSH Park or Seeed.
Different topic: I have yet to crack it open but I know one of my H batteries is really weak. I'm pondering just gutting it and going JK, but also considering replacing the contactors on the "good" battery with something like InPower SSC3-200B. I might have missed a video but have you looked into solid state contactors for the KS2 batteries?

[@parkhopper3627]

Thanks for your comment and I will keep your offer in mind. H to JK is a good idea. I am in the process of converting two Ecotreks to JK's and should be done in a month (waiting for parts). I have looked into solid state contactors briefly. Replacing just the contactors is not a good idea just to avoid the high overhead. Solid state contactors are expensive and uni-directional. The Ecotrek contactors are bi-directional and robust. The weak link is not the contactor but the complex discrete electronics and all the wiring connections. For half the price of a solid state contactor, you can buy the JK BMS that has a bi-directional solid state contactor built-in.