From what I can tell it takes time. I think it was several days after I made beds to improve the camp (which also took several days before I got the option it seemed) that I finally got the chance to rest with tank.
I believe you need improved beds, to have a decent relationship with him and Scar, and to ask Scar about Tank -that's how I got that. I also had fucked Tank, which may be a requirement. However, I did get the prompt right after talking with Scar about Tank
