I am not very familiar with the Manistee NF, since the last time I camped there was about 35 years ago.
You might want to download a copy of the Motor Vehicle Use Map (MVUM) from their website for the lower part of the Manistee NF.
MVUM - Link While this map is primarily of use for ATV riding, it shows all the National Forest System roads and specifies what type of motor vehicle can go there (RVs, TT, 5ers would be in the highway vehicle category). It also has the following statement, which at least on some other national forests, was done to still permit boondock camping along some roads:
Motor vehicle designations include parking along designated routes and at facilities associated with designated routes when it is safe to do so and when not causing damage to National Forest System resourcesThat "facility" word was put in to address some of the wide spots adjacent to, or at the terminus of dead end roads where RVs often boondock.
