Aux. Gas Tanks

[GMC_man]

Looking into buying one of these truck, cause there so awsome, But have a few gas tank related questions. I'v seen the long beds with the steel square door on the side and have been led to belive these are factory installed aux. tanks. I'v also herd these aux. tanks where offerd on both left and right configerations or both(for a totaly of three tanks). Just wondering if anyone know's anything about these tanks such as was it availbe on short beds and could you get it on both left anf right sides for a total of three tanks if desired? also what years where these availbe all years?

Thanks, and sorry about the spelling