Length isn't the only consideration, height is something that must also be kept in mind. Many forest roads, like those found on the North Rim, have low hanging tree limbs that also must be dealt with. I seriously doubt a high profile 5th Wheel or Motorhome can navigate down some of the forest roads on the North Rim, at least the ones I've been on. Otherwise, you may need to put a friend on the roof armed with a chainsaw to deal with this issue.
When it comes to RVs, my advice is the buy the smallest RV that meets your requirements. Bigger isn't always better, especially when it comes to boondocking.