The "14 day rule" has no relevance to off-road capabilities of your rv. There are all types and locations of dispersed camping sites. Some are well off road and inaccessible to other than small, high clearance vehicles. Many in desert areas, specifically in southern Arizona, are flat, adjacent to highways, and are accessible with no more than a few yards of off pavement driving. There are some NFS locations that require considerable "off-road" driving, but which will accommodate larger RVs in the camp sites.
Availability and accessibility of dispersed camping locations all depends on where you are. Use the following link to find sites, access, and descriptions. We frequently boondock in our 40 footer pulling a Jeep and have used this site for years and find it very helpful finding all sorts of sites.