How to remove wash anthracene oil/creostae oil from stainless steel tanker?
Anthracene oil (CAS registry number: [90640-80-5]): Both European Chemical agency (ECHA) and US Toxicology Date Network (TOXNET) classify anthracene oil as:

A complex combination of polycyclic aromatic hydrocarbons obtained from coal tar having an approximate distillation range of $\pu{300 ^{\circ}C}$ to $\pu{400 ^{\circ}C}$ ($\pu{572 ^{\circ}F}$ to $\pu{752 ^{\circ}F}$). Composed primarily of phenanthrene, anthracene, and carbazole.

Anthracene oil  is also classified as a carcinogen the risks to human health and to the environment as well (though the relevant symbol is not issued).

Creosote oil (CAS registry number: [8001-58-9]): Creosote oil is a synonym for coal tar creosotes. It is a distillation product of coal tar. Creosote oil is an oily liquid consistency and range of color from yellowish-dark green to brown.  It is a mixed compound, composed primarily of aromatic hydrocarbons, anthracene, naphthalene, and phenanthrene derivatives. At least 75% of the coal tar creosote mixture is polyaromatic hydrocarbons.
Creosote oil is heavier than water (specific gravity: 1.07-1.08) and has a continuous boiling range beginning at about $\pu{200 ^{\circ}C}$  (range: $\pu{194\!-\! 400 ^{\circ}C}$).  They are slightly soluble in water, but miscible with alcohol and ether (Ref.1).
Koppers' Safety Data Sheet has following warnings for creosote oil:

Hazard Statements:

Causes eye irritation.
May cause cancer.
May cause respiratory irritation (No wonder your servicing person had severe respiratory problem when he went inside the tank).
Very toxic to aquatic life.
Toxic to aquatic life with long lasting effects.


It also listed following precautionary statements for prevention:


Avoid breathing dust/fume/gas/mist/vapors/spray.
Wash thoroughly after handling.
Wear protective gloves/protective clothing/eye protection/face protection.
Obtain special instructions before use.
Do not handle until all safety precautions have been read and understood.
Use only outdoors or in a well-ventilated area.
Avoid release to the environment.


I put all this effort to show that how dangerous your situation is and how ignorant you have been. Your first mistake is not receiving all special instructions for handling these kind of chemicals from responsible people (see above bold-face precaution). In your case, I assumed the manufacturer of the chemicals should provide these details. What you should have done now is go back to the manufacturer, and request the instruction for clean-up procedure safely. And be aware that whoever the person following that instructions, should wear with all personal protective equipment (PPE) required, specially suitable breathing equipment.
Alternatively, since the both oils are viscous, you may slant your tanks backward to make them drain out to safety container (for long time) before starting to wash them (assuming drainage is in the back). Once drained, a person wearing PPE can go inside and wipe out inner walls with cloths soaked with alcohol several times, since both oils are miscible in alcohol. Also, kerosene or gasoline or even coconut oil may work as well (please dispose those cloths accordingly). Finally, you should use appropriate detergent (or soap) with water to remove trace of remaining hazard chemicals (if there any) and washing organic material used.
References:

B. F Hibbs, J. Georges (Agency for Toxic Substances &amp; Disease Registry), Toxicological Profile for Creosote (updates):  Wood Creosote, Coal Tar Cresote, Coal Tar, Coal Tar Pitch, and Coal Tar Pitch Volatiles; Forwarded by Dr. D. Satcher; Prepared for US Department of Health &amp; Human Services, 1996.