How do you plan on deconing the tank, before or after its out of the ground? You can try and scratch the fiberglass off with the hoe before you yank the tank. We try to beat the tank out of shape a bit and try to get the fiberglass to separate. I promise, it will make a big ass mess if you are able to get the coating to come loose.
Most of the time it doesnt come off fast enough to be worth our time. We will uncover the tank, make sure its empty. Then vent with our venturi nozzle and air test. Then we use the rivet buster with our tank cutting bit and open it up in the ground. Do our normal confined space program, clean and drum sludge. Then use oil dry and scrub the tank down. Then we finish digging and removing the tank, then load it out on a flatbed and send it to the scrap yard. We will take the deduct on the metal due to the fiberglass coating still being there.
I doubt you will want to risk selling a tank that has been in the ground since at least the early 90s.