A lot of National Forest campgrounds will say less than 22 foot. That is because most NF campgrounds were originally set up for tent camping only and not for RVs, thus it was assumed most would be occupied by cars and trucks with tent setups. In reality they will usually accommodate any Class B but many were not designed level enough for sleeping. You just have to check them out.
In Minnesota and Wisconsin I've found many to accommodate my 24 foot Class B but the upper Midwest is mostly level ground. I visited a NF campground in Georgia at the beginning of the Appalachian trail and though several campsites were unoccupied and the length was accommodating they were not level. I ended up overnighting in the adjacent trailhead parking lot.
2015 Advanced RV Ocean One Mercedes Benz Sprinter
Previous Class Bs:
2011 Great West Van Legend Sprinter
2005 Pleasure-way Plateau TS Sprinter