I was just there a couple of weeks ago, just after that last big rain. Some of the forest roads were so muddy, all four tires on my Ford Ranger were spinning for miles at a time climbing out of some the canyon logging roads. It should be much better now.
My work was south of Downieville, so I'm not familiar with the roads on your route. In general, the forest roads are not that steep. The steepest one I found was Galloway Street leading south out of Downieville. If Google Earth isn't working for you, try using the "terrain" layer in Google Maps, which will give you the topo map.
I found the forest rather difficult to navigate in the area south of town. Most intersections and roads have no signs, and my GPS could not lock onto a single satellite with any constancy. A good old fashioned compass and the ability to dead recon is a valuable asset out there. I do not remember seeing any signs restricting OHV motorcycles. The whole day I was in the forest I only saw about 3 vehicles, and none would have cared either way.
It doesn't look like Sloat has fuel, so if I were you, I would just get fuel in Quincy.
Downieville: I stayed in the Riverside Inn in down town. Every room overlooks the river, and they have bagels with salmon and locks for breakfast. They also have covered parking, but I'm not sure if it is for guests since nobody used it except the mountain bikers. The older gentleman who serves breakfast knows the local roads well, and can give you advice about the area.
Restaurant hours are slim, so plan meals ahead and use the motel fridge.
2009 Aprilia Dorsoduro ADV750