Fuel economy on Thor Super C

Wow, I didn't realize how small the DEF consumption was, I guess I always assumed it was more. Does this DEF volume only apply to the Ford 6.7L that Thor is using on their 2 New F-550 Super C Motorhomes or just in general for a modern diesel of similar displacement?

Is DEF consumption directly related to diesel fuel used, or are there any other variables to change DEF usage?

Thanks in Advance.