Reverse polarity protection using back to back NMOS
This is more or less what I have in mind. When I use back-to-back FET's I almost always start by pulling the gates to the sources with a resistor. This is a foolproof way to make sure they will be off in normal operation. Then I go from there.
In this case, D1 is needed to make sure that during reverse polarity connection no current flows through the body diode of M2 into the control circuitry I assume is connected to G. If that were to happen, the MOSFET's could be damaged as well as anything connected to G.
In addition to what I have shown, you may want Zener protection in parallel with R1 to make sure Vgs never gets high enough to damage M1 and M2.
Note that if and when M1 and M2 fail, you will have a short circuit across the power rail. So you should give some thought to the consequences of that to make sure they are acceptable. It is a foreseeable failure, not a one in a million freak occurrence. So the consequences need to be acceptable, or if not, you may need to add a fuse or some form of detection, etc.



simulate this circuit &ndash; Schematic created using CircuitLab