I've never heard of tadpoles in a grey tank before, but I guess it isn't any more out of the realm of possibility as a snake or rat crawling out of a toilet in a sticks and bricks house, which I have heard of before.
Tadpoles are not parasites, they are amphibians. No harm with them being in your grey tank. They eat algae and rotting plant matter, so if you have none of that in your grey tank, then they should die on their own. If they mature to frogs, they will most likely hop down the sewer hose to freedom. If you are set on getting rid of them, like all amphibians, they do not tolerate chlorine well at all. Even a tiny bit of chlorine will kill them. Close the grey tank, put a couple gallons of water in the tank and then a couple tablespoons of bleach. Let it sit an hour or two and they should be dead.
We live in our camper for 4 - 5 months at our seasonal campground. We too leave our grey tanks open all the time. With the 4 of us, we'd be emptying the grey tanks (we have 2) on a daily basis. No point in that. Enough water moves through both tanks on a daily basis to keep the tanks clean. Towards the end of the season, I usually fill them, add a cup or two of bleach and let them sit. Then empty both. Never had issues the last 4 years doing this.
-Michael