Adding an auxillary tank fill to a 2021 3500 SRW

[06 DIESEL]

So about two years ago I installed a 50 gallon tank in the bed of my megacab. I never finished the hookup of it and now I want to finish it. Biggest reason was the metal pipe that is the filler neck vs rubber like every other tank I’ve ever had down. The question is, can I T into the what I think is the vent/evap line and use that if I only plan to fill the tank out of the Aux tank when needed for driving on long trips with the trailer? Is this the vent line? Am I about to screw up everything that works so well when filling the truck if I do this? Do I just need to bite the bullet and cut the metal filler pipe and get it done properly?

 

[06 DIESEL]

Bumping this to the top to see if anyone has any recommendations before I go to cutting?

 

[4x4dually]

Most transfer tank fill kits tee into the large main filler tube. If you are trying to shove fuel down the vent and the main tube is closed, I'm not sure how well that will work.