....cut......
Notes: the isolator switch on the battery box is manual.
The link below is the upfitter connector and it does have a 50 amp fuse But it may stay on independent of the key. which might drain my starter battery. Unless the isolator takes care of that.
https://flic.kr/p/oXcFoP
Thanks for sharing this information. And also on the portable battery unit. I had looked at a similar unit in the past to run a 1000 watt inverter for small microwave.
Anyway, a few random thoughts:
Where are you going to place the house battery? I ask only because the upfitter connector box is by the sliding door, which may not be the best place for battery. Depending on where you place it, it may be easier to tie to power at vehicle battery.
Because connector is reportedly always hot (has power), you definitely don't want to forget to disconnect the house battery while dry camping regardless of how you get power to it. Personally I'd even go one step further and initially check with volt meter and/or conductivity meter to make sure than when isolation switch is open that the house battery is completely disconnected from vehicle.
The specs you linked show minimum cable size as 10 mm^2. That answers your question about cable size for 50 Amps. It does not hurt much to go bigger except for cost, weight, and over current protection in case of accidental grounding. These are all next to trivial factors.
Regarding the maximum current that may possibly flow to house battery during charging, that's a tough one to answer -- more like guess at. If you park for long period and your house battery gets fairly low, while your vehicle battery remains fully charged, then when you start the van and close the isolation switch a lot of current (as in Amps) can flow from alternator to house battery. The larger the house battery the lower it's resistance and the greater the chance you could exceed 50 Amps. The alternator could be trying to maintain 14V while the depleted house battery is at 11 Volts. Without restrictions of some type I can see going over 50 Amps under some conditions.
It's possible your house battery box may limit current somehow since the connector is rated at 50 Amps. Or you may be able to find a simple gadget to limit battery-charging current (an inline resistor would do it but it's highly inefficient -- but may be included in the house battery box).
You can always try calling the house battery box manufacturer and ask them what the maximum charging current may be. And while at it, I'd ask them if it prevents current from flowing back to vehicle.
One final thought -- house batteries often have a direct connection switch to bypass automated isolators. This is used in case your vehicle battery is dead (like you left vehicle lights on) and you want to use the house battery to jump start the van. Since your isolation switch is manual, just closing it will likely do the same unless RAM or battery box prevents current from flowing backwards. What can easily happen when you forget to open the switch every single time is that if your starting battery is weak at all then the house battery will help in starting the van. And that can easily exceed 50 Amps.
I mention all this because most of the time it all works out fine, but occasionally unexpected things can happen that are hard to figure out. What you have is a simple system yet I can see potential problems if everything is not compatible.
Sorry for the length, just sharing some food for thought.