Welcome aboard, and this is a good place to get information. Be patient.
I do not know your exact boat, but in general, it has three types of tanks. Fuel tanks; water tanks; and sewage holding tank ("Black Water" holding tank - your boat is old enough that it probably does not have a "Grey Water" holding tank for sink/shower water).
90% chance your fuel tanks are 'saddle tanks' meaning they are outboard of the engines. This places them beneath the side-decks. Usually there is a deck-fill plate in the center of the boat on each side, above the fuel tanks. DIESEL ONLY
Water tanks are more variable in placement. Half the time they are under the aft cabin master bed or nearby. You probably hold 150-200 gals of water which is tough to hide, even in two tanks (which is common). HINT - water tanks are usually plastic, and the water pump is often fairly close, and there are often valves to isolate the two tanks. Even if there are two tanks, likely just one deck-fill, and probably in the back of the boat near the tanks.
Black water tank - this is the smallest, probably in the 40-gal range. Usually in the belly of the boat. There will be a single deck fill, often in the forward half of the boat as the tank is often in the forward 1/3rd of the boat, centrally located between the two heads. You can follow the 1-1/2" discharge hose from each head/toilet. There is often some valving to direct the waste either up through the deck pump-out fill cap, or overboard (perhaps via a macerator pump).
So Job #1 is to find the tanks. The deck fills will probably be nearby and overhead.
Good luck -
Peter