In my experience, the 29 ft length limit is usually for the turns and roads in a park. A travel trailer or a fifth wheel can bend around some of those tight turns much easier than a comparable size motorhome. Many older campgrounds, such as El Morro National Monument nearby, have great roads but only one spot fit for an RV (and a short one at that).
For example, I called the Forest Service about Madera Canyon campground in AZ. I was told there was a 27ft length limit. I parked my 27ft trailer at a friend's property and drove my pickup truck in.
Yes, there was a concrete flash flood ditch across the road that was also on a sharp corner. It would have been a problem for a motorhome, especially one with a long overhang in the back (This is most of the shorter ones) or a rig with little ground clearance.
Lo and behold, sitting in the host spot was a travel trailer the same model as mine. THEY got in. There were a three sites that I could have fit into.
Some of Joshua Tree National Park's campgrounds have a 24ft length limit, and you better take it seriously. I couldn't have gotten my extended cab, full bed pickup truck into most of those sites and some of the turns were a challenge.
However, when in doubt, SCOUT IT OUT! ;D