When camping in an RV in national parks the national parks have an average permitted size of 27 feet for RV's in most camp areas of the national parks.
Although some designated areas of the national parks can allow up to 40 foot size RV's but the average size is 27 feet so if you have an RV that is 27 feet or less then you can usually easily get into the national park without problems.
If your RV is over 27 feet you usually need to have reservations for an RV site because they're usually filled up if you show up without a reservation.
But if your RV is 27 feet or under in length then you can usually just pull into the national park and camp anywhere which is known as dispersed camping.
But you won't have the full RV hookups or any RV hookups at all and will be boondocking it without any comforts other than your RV.