National Parks and Trailer Length - What do you recommend?

A trailer with a box with a max of 22 feet or 25 feet overall almost always fits in some NPS camp sites. Sometimes you have to disconnect your truck.

Large spaces a few and far between and in great demand. Many people camp outside the parks and commute with larger RVs especially if they want hookups.

We are national park fans. We have a 30' class c and often have to "shop" to find a spot in within the park sites. Our previous unit, 26', was lots easier to find a home for - we've been doing this for 40 odd years and seen little change in NPS campgrounds, the shorter the better.

It's something you will have to research park by park using their websites. Some, like Big Bend NP in TX, have ample camping for big rigs. Others, like Mt. Rainier NP in WA, are very limited. This is a case where one size does not fit all.

Food for thought there are only 48 national parks in the lower 48 and at least 8 of them have no RV camping - do you get a small RV just for that

There are more parks run by the national government but only 48 are designated as true "national Parks"

National Parks are only a sliver of a small fraction of the public govt run campgrounds in the US. Are you honing in strictly on NPs?
Either way, in general, you should be good with backing and Jack knifing trailers anywhere, not just NPs.
If there's any skepticism there, a shorter Moho and toad are the easy button for fitting into campsites. You can't make a truck and TT shorter until the TT is parked.

Pretty much any modern setup. Crew cab full size anything and any square box TT (leaving out the teardrop type little campers) will but you at 40' combined length as you've seen.
Go find a 20-24' trailer and drag it around for some experience and see how comfortable you are with it is my suggestion.

That said, the appeal for me would be the same. Have a big comfortable vehicle (truck) to tow with and daily drive. Have the option of way more power than a Moho (except for diesel super c and pushers), and only have 1 vehicle to maintain.
As far as towing, a little toad behind a big box will tow "easier" apples to apples than a med size tow rig pulling the big box.
There are plus see and tradeoffs to any setup. Just consider both equally when making the decision.
For instance, a V10 or big V8 gas Moho will tow a small car/SUV fairly imperceptibly except for the gas pump bill. Stick a 4 door jeep or vehicle in the 4-5klb range behind it and it starts becoming a boat anchor with a gas Moho.
Same size living quarters with a pickup and TT and you can have a pickup that tows with a lot of power to spare, but you're pulling an 8x10x___ billboard/sail when the winds pickup or the road grades do.

Idk how much room you're looking for, but if you paired a short short bed half ton with a short TT, low 20' range, with single or multiple slides, you'd have a lot of living space and that's a combo I'd be able to shoe horn into any place a med size C with toad could go.
Other things are, do you value being able to go fro. The cab to the "house" directly? That's a cool feature of a Moho compared to a TT.

From 12 years of full timing at NP, COE, BLM and NFS "parks" with TT and 5er if you stay under 30 feet you will be OK. 30 Feet and over you need to make reservations 6 months in advance.

P p I n e has it right. Our first tt was a 25 foot Airstream. It was the perfect length to be towed by a half ton truck and also for space. Longer and shorter have both their positives and minuses. Be aware you are going to tow it park it maneuver for fuel etc.

Too many “no goes” for me with any TT over 25 feet. Currently have a 22 foot Winnebago TT and some NPS, BLM and USFS sites are still not long enough.

Every National Park measures their campgrounds differently. A lot of the time a campground maximum length is set based on roadway restrictions. A good example is Mount Rainier. Individual campsites might take a longer vehicle, but the ability to get a trailer around right spots in the CG restricts the maximum size.

I’ve pulled a 39’ TT (tip of hitch to rear of sewer tote carrier) with a 22’2” long pickup across 46 states and 45,000 miles in 4 years and 5 months. I’ve been able to get into almost every CG I’ve wanted. Of the NPS operated CG I’ve visited, most are National Monuments not National Parks. Frankly, I stay a lot more at COE federal campgrounds, BLM and Forest Service than NPS parks.

The biggest hassle is hooking up/ unhooking. I often travel with friends with Class A units. They are always faster/ spend less time than I do. Of course they have automatic levelers, I don’t.

Since sites with hookups are very rare in National Park CG, I also have to unload and setup my generators. Theirs are built in.

I would consider any pickup less than a 250/2500 inadequate for the type traveling you are considering. Diesel if you can afford it.

My preference if I was starting this journey over would be a 30-35 ft Class A. A small diesel if possible. We had a car that would have been a good toad, but we sold the 30-35 mpg potential road to do 35-45,000 miles of day trips a year at 13-16 mph in our 2500 pickup.

When I get near cities in many states I find the pickup size limits where we can go and what we can do/ see more that the trailer size.

As mentioned above, it is all about trade offs.

Another key is how much of the year you will do this. It is one thing to take a nice sized trailer out to a favorite boondocking site three or four times a year.

Completely different to spend 85 days in Utah, moving 20 times. 20 times setting up the generators and solar battery charger. 20 times tearing them down.