Every National Park has RV length restrictions. There does not seem to be any consensus anywhere, nor an authoritative resource to tell us how sites are measured. Typically the length restriction will be for camper AND tow vehicle, so the allowable length for a trailer will be shorter than for a motorhome by about 10 feet. Our best advice is to take a look at the list below and use it as a guideline, but not as a bible. Nor should you use ANY online resource as authoritative. We strongly recommend that you do the following:  

Call The Park: We have found that at the end of the day, no matter where you are trying to camp,  you ALWAYS do better by speaking to someone right there at the park. You will get first hand information that will often times contradict what you read or hear online. For instance if you are just staying overnight and do not wish to unhook, make sure you can get a site long enough for the whole shebang. 

When we were staying out at Yellowstone last year, we stayed outside the park, at Henry’s Lake State Park (AWESOME park.) During our 2 month visit, we stopped around to see several of the Yellowstone campgrounds. Keep in mind that this was mid summer, when anyone would tell you it is next to impossible to get a site within the park.

While visiting the Mammoth Springs area, we noticed that the campground there had quite a few empty spaces. We stopped to speak to the ranger at the gatehouse, and she told us that they had been experiencing quite a bit of vacancy over the last couple of weeks. They usually got several walk-ins during the days, but each morning many left. 

We also noticed that, even though the park has a maximum RV length of 30 feet, there were a large number of sites that would, and WERE holding RV’s much longer than that. The ranger told us that the restriction was there, but they often put larger rigs into the park without a problem. 

Another story: When we visited South Dakota, we stayed at a Angostura State Recreation Area, a huge, beautiful facility on the Angostura Reservoir. The campground offers electric only sites at VERY affordable prices. When I called them on the phone to get some information, I got some surprising news. 

The ranger who I spoke to told me that they did in fact have a few full service sites that were used by the on site camp hosts. Since it was late in the season, some of the hosts had been released and left. So there was a fully paved ADA site with full services and a built in stone fire-pit available if we wanted it. (We wanted it) I think we paid about $25 per night for the site. All because of a phone call. 

The point is this: call the park. ALWAYS. Get first hand information from someone on the ground, who does this every day. You never know what will happen.