Based on my Tembo experience for the peeps considering a Long Ranger, I would suggest figuring out a way to keep the Aux and Main isolated except for the fuel transfer point from the AUX to the Main. Resulting two separate and isolated filling points and paths, leave the OEM alone and add a second fill point for the AUX- Eyeballing it you could source a second OEM fill neck and with not a lot of work fit into the current fuel door. Ending up with the OEM and AUX sitting next to each other. The duel fill setup of the long ranger will cause issues with the OEM EVAP as it will introduce a leak into the EVAP system.
Got to love solid wifi on cross-country flights it gives one time to catch up on mud.
As you see in the above photo the two fill paths are linked at the point the nozzle enters, hence the tanks are linked; they need to be separated, two gas caps, two fuel filling paths one to each tank. Linking the two fuel tanks the OEM evap will try to pressure and vacuum test the volume of the OEM tank and fuel filling lines plus the Aux tank and fueling lines- it will time out and throw a code. The fuel transfer pump should serve as one-way valve/check valve of sorts separating the tanks at the point of fuel transfer.
The US petrol spec has an active evap system that goes through a robust selftest and certification.
The Tembo vents into the OEM, I see where you are heading-
I'm interested in the 70L tank. I have LRA tanks in my 80, one of the biggest expenses was air freight and it was the same price for shipping one tank or two tanks. If everyone is ok with waiting a little longer, sea freight is cheaper. I have a relationship with LRA and can get a discount with them, but it looks like everyone is more interested in the Long Ranger options. Their 90L high clearance tank looks nice. Add me to the list.
I like @Forrest Murphy's suggestion to avoid evap code issues. Two separate tanks, and have the transfer pump dump fuel from the aux tank into the main tank with a one-way check valve. If there's no evap checks on the aux tank then when the main tank pressure tests it will pass, and the aux tank won't open the check valve except when dumping fuel. Side benefit is that you can choose to just refill your main tank for driving around town, and only fill the aux tank (and carry the extra weight) when you travel. I assume the aux tank would require a charcoal canister if it's completely separate though, otherwise any vapor would get "dumped" into the main tank during expansion.
I was thinking along the same lines, just need to define a safe place for a second fuel filler inlet
What would be good to know if the bracket made in Forests thread for the Canister will work in both applications. If so maybe we get some of those made and as a first step move that as it is a requirement for either option
The best place would be on the LH side of the vehicle, where a fuel door belongs on a US truck!