I agree that you can not ride anything that is not street legal in a National Park. National Forests are another matter, and there I would expect to be able to ride, providing it is not under a fire restriction, or other local reasons to prevent you from riding.
Some of the national parks I have been in, you can not back in, then use the rear ramp. Some you can. Most BLM Land, there would not be 'assigned' parking, or limits to length. I am going to make the same difficult decision, and might end up with a toyhauler motorhome to avoid the backing in after unloading problem. It is so easy to just raise the jacks, pull out 10 feet, then unload or load the toys, then pull back into the sight out of the roadway. Or drive into the site, and fold down the ramp into the roadway long enough to load or unload something.
I fell in love with the 45' long Heartland with two bathrooms! It would allow easy wheelchair access to the lower level tub/shower. However that length! And I want room for the wheelchair in the truck, so it would mean converting a Econoline van to a dual wheel hauler, or some other sort of truck, such as a regular cab truck with 200" wheelbase (yes they make a cab and chassis with 108" cab to rear axle length) and then install a 72" long sleeper cab (like used on big rigs) and still have about 5 - 6 feet left over for a small utility type bed on the very back. Yet that would be one REALLY long truck, pulling a equally long trailer, with limited ability to make u-turns or pull into a tight National Park campground. It would be great at a BLM location, or other open areas!
You might find a need to have a smaller trailer in the 25' length for camping in the National Parks, then take the big trailer to the BLM or other off road areas. Just a thought?
Good luck!
Fred.