I wasn't aware of any plastic tanks in the Grady line. Maybe so. I know Florida Marine is still making aluminum tanks for Grady, saw a bunch getting ready to be delivered to Grady factory while I was there picking mine up.
My tanks were original to the boat and both aluminum. Some guys have better luck and a longer lifespan than mine. I contribute the premature death of mine to lack of care and proper flushing of them in it's first 13 years of life of living in a saltwater environment.
If it was me, I'd pull the floor panel up and give it a good wash and rinse just for piece of mind if you just bought it. At least you'll have a starting point moving forward. Then again, if it truly is plastic, you might not want to. I'm OCD trying to keep things clean and in good condition, me, even if it is plastic, I'd still pull the panel for a good wash and rinse.