Hey Gian -
Route assistance: See
MDOT bike use map. The Thumb Area map looks like M-25 (the shoreline road) is good for riding, at least by MDOT standards. I agree w/ Machka - route planning/mapping is half the fun for me.
Camping:
Mich DNR site lists campgrounds by county. I like the state forest campgrounds more than state park campgrounds - much more rustic (most have no power at sites = no motor homes). Unfortunately, I don't think there's many in the thumb area - unless you plan to head a bit farther N & W. Or,
private campgrounds often are willing to cut bikers a deal to camp on the grass in a corner. I
wouldn't try stealth camping in the immediate vicinity of a state park - it's posted in these areas that camping's allowed in designated sites only, & the ranger routinely checks out the area looking for sneakers. Surrounding state land (outside the park boundaries) is OK.