All the trails that are part of the RSR have been ridden with 28mm slicks.
And all of the same trails have been ridden with 3.0 plus tires.
And both are fun! It just depends on what you're looking for out of the day, and what your on-bike skills are.
Conservatively, fatter tires with more knobs are better. Especially by the end of a long ride when you're tired. At that point a little bit more traction on the dirt may be better than a little less rolling resistance on the road. This is especially true for the 90 mile route.
If your bike will fit it, a 40mm tire is great. Something fast rolling, but with some outboard knobs like a WTB Nano40
The Strada Rossa is not a race, so being a few minutes slower due to a chunky tire shouldn't ruin your day. Washing out on a loose corner and going to the ER will. Choose accordingly!