I've ridden the Katy trail in Missouri quite a few times on just regular road tires. Mostly graded pea gravel.
There's a canal trail around here that I also hit with whatever I have on my bike.
The canal trail has one steep climb (or descent) which can be a bit hairy going either way. I just don't feel safe for hard braking or sharp turns on the gravel.
Anyway, I suppose a lot would depend on what your roads are like. If it is mostly straight and flat, then I'd just ride your normal tires. If you have a lot of steep hills and tight corners, then maybe consider something else.
I do have one road that I'm wanting to ride that has about 2" to 3" gravel on it, so the road bike is out.
The Schwalbe Marathon Plus 700x25 has a moderate amount of tread. But, I think it would be overkill for a single ride.