Contact rating of 40a is way too small! Duh... PAC/Stinger 200 amp relay is a minimum. It must have a rating equal to or greater than the alternator's rating.
You are quite right that PMOS has a higher rdson, thus after reviewing 50+ spec sheets I found NMOS to be a better choice. I will be using a voltage booster (MAX1044) to get more positive voltage. This is part of a microcontroller driven coulometric charge controller which also takes into account temp and voltage. Coulometrics accounts for amp hrs in vs amp hrs out to create an accurate guess of how much charge is needed. Battery voltage, when evaluated based on temp and current load will also be taken into account. In case of transistor or battery overheating, I can also just disable the gate and let the existing over-the-counter diode isolator take over for awhile.
The alternator's regulator holds the main battery voltage at ~13.8V. If the main battery doesn't need much current, the isolator drops to min 0.7V so 14.5V is available on the alternator output, but the other isolator diode will drop between 0.7V-1V to the deep cycle. That's not enough voltage for a bulk charge, it's more like a trickle.
I'd like to run the accessories/fridge all day, and if I'm camping and not driving around, I want idle the car for as little time as possible to restore it every day or two. Stupid to run a 200 hp engine and a 160 amp alternator to only produce 10 amps into the battery! That huge battery should be able to take 50 amps if it's not in overcharge. Charging at 14.5V is very healthy and a far more appropriate charge voltage.
But, also remember I want to be able to run a 75 amp inverter load while the engine is running. The battery will still regulate its own load and may be pulling 30 amps, so the circuit may have more than 100 amps on it even though the battery isn't drawing that.
Now I do have some huge 240 amp Schottkeys:
https://www.electro-tech-online.com/custompdfs/2004/06/245nq015.pdf
The reverse leakage should not be a problem, as I can turn off the MOSFET under those conditions. However, getting bulk charge currents is a matter of tenths of volts, I will already be dealing with the MOSFET rdson drop and I don't think it'll perform with an added Schottkey drop. Also, the Schottkey's in a large, weird package and I don't intend to build a 25W+ heatsink for it. Not a good idea unless the MOSFET just can't be done.
Brent, do you have any links or remember if there was a specific term for this arrangement? I need to know if the technology has negative consequences associated with doing this, the spec sheets never list properties for doing this.