In National Park Campsites the longest permitted RV size that can fit and is allowed to go into National Park Campsites are RV's and campers that are 27 feet and less in length.
If your RV or camper is over 27 feet in length then it will not fit into the National Park Campsite nor will the RV or Camper be permitted into the National Park Campsite.
So if you plan to camp with your RV or camper in National Park Campsites you should have an RV or Camper Trailer that is 27 feet in length or less.
If your travel trailer or camper or RV is longer than 27 feet then you'll have to go to other campsites to camp because you won't be allowed into National Park Campsites with a larger RV.
So the ideal length of RV size for fitting into National Park Campsites is RV's that are 27 feet or less in length.