Reality of RV length restrictions?

Get the RV you like and make the best of it. You will find a few secluded wooded sites and for a few you will be at Yogi.
Either way will be a compromise.

The reality is that there is no consistency.

In general, the larger the rig, the more places you cannot take it.

No way we can get our 36' TT into Mount Rainer NP campgrounds. Personally, I think I can, having walked the main two CG when they were closed. But I've been told on this forum and at the park that my TT would be stopped at the entrance, measured, and I would be turned around and sent out of the park.

Ok, there are several alternates in that area, and we enjoyed several day visits to the park during ouf time in that area.

Many full-timers prefer to stay outside the parks in order to have full-hook up facilities. We stayed outside Yosemite due to lack of available spots in the park.

Two friends contacted the folks at Yellowstone 18 months ahead of their planned visit, and were able to work out a nice 2 week stay, dry camping in their 44' tag axle motorhome, in the Bridge Bay CG.

We had two weeks notice before our route changes made a short visit to Yellowstone possible, so we stayed in West Yellowstone.

The places were worth the trips, flexibility is the key, and for really popular parks, try to plan far ahead.

No matter what size camping unit you choose, there will be some places it will be too big to get into. Heck, I have a favorite fishing lake I am not man enough to carry a sleeping bag and a one man tent up the trail to reach it. So we camp at the bottom of the cliff and climb up in the morning leaving the tents, stove and cooler behind. You do the same thing with any RV, if you can't fit in one location you stay in the nearest one you do and go from there.

I doubt you have a 12 ft truck, mine is 23 ft. But as noted above that is probably not a big issue except for your turning radius. There are some NPS and USNF cg that you will be able to get into but more that you will not fit in. We had a 35ft fiver.

You will be able to get into most but will be limited to the number of sites for rigs that size. Were able to visit Fishing Bridge in Yellowstone with a 36 foot 5th wheel and a double cab short bed tv. It was tight but doable.

I have full time since 2006 with a 30 foot travel trailer. The overall length on a 30 foot travel trailer is a little longer than the overall length on the 30 foot 5th wheel. So last year, we purchased a 32 foot 5th wheel. This is about the maximum length you can use to get into many national parks. Even with the 32 foot 5th wheel I have to cheat and tell the reservation people that it is only 30 feet.

My preferred camping is in national parks, BLM land, or national forest land. I very seldom camp in private campgrounds. I would strongly suggest that you try to find a trailer that is very close to 30 feet maximum length. Anything longer will limit the number of campsites in national parks that you will be able to camp in.

Having said that our last 5th wheel is 36 feet.

The only real site length factor is the trailer since the truck detaches and can be parked separately. Most public and private parks have at least some sites that will easily accommodate an RV that size. Some length restrictions are related to site access rather than site size though, with narrow roads and trees, boulders, etc., the limiting factor.

Many National Parks do not have any accommodations for large RVs. That includes a lack of hook ups and suitable facilities. Typically there will be RV parks or commercial camping areas immediately outside of National Parks that many prefer. Big parks such as Yellowstone have a mix of campground types. With a reservation well, well in advance you can even get hook ups and there a quite a few choices which will accommodate a large RV but without hook ups.

Since you have been camping for years you should be familiar with the types of camp sites available in various locations. The bigger the rig the more limited the choices, if any, that will be available within National Parks. You will also find fewer if any choices for trees and privacy.