Not a lot of off road opportunities in Yellowstone, there are just so many tourists it would get destroyed. there is a several mile dirt trail in the north end, but not much else. There is a good dirt road (Ashton Flag Ranch Road) located half way between Yellowstone and Grand Teton with some good primitive camping.
Due to the size of the park, we find it convenient to stay within the park for at least part of the trip so we don't spend so much time travelling in and out. Our campground of choice in Yellowstone is Madison Campground, located in the park itself on the west side. This campground has nice sites, they allow tents or Rv's and is in a good location for exploring. The town of West Yellowstone is not too far if you need supplies. West Yellowstone itself is overpriced and hard to get reservations in as its is so busy. As mentioned Yellowstone is huge and takes several days just to a cursory tour, make sure you allow plenty of time there. Typically one of the best parks for animal sightings, getting up very early will provide the best opportunity to see some of more elusive critters. While the whole park is littered with buffalo, Lamar valley in the NE corner of the park has always been the best viewing area for us for Bear, Wolf, and Moose.
In Grand Teton, Gros Ventre Campground at the south end of the park is very nice with quick access to Jackson. Be sure to check out Moose Wilson road leading out of Moose Junction if you are looking for moose sightings, parts of this road are often closed in late summer due to bear activity, but you can often see moose on this road. There are a few off pavement trails in Teton, and they tend to be far less travelled. If you hate crowds avoid the Jenny Lake area its always absolutely jammed.
Glacier Park is gorgeous and travel over Going to the Sun Road is a must do, however be aware that max vehicle size is 8 feet by 21 feet (think there is a height max of 10 feet also). They did have the road closed at the east end due to Covid this year, but you could still go over and back from the west. There are some primitive camping options on the West side of the park up by Pole Bridge. Fish Creek Campground inside the park by the west entrance is a nice campground on Lake McDonald (which is an amazingly clear lake). We spend a lot of time in this park and the surrounding areas provide a lot of overlanding opportunities. If you feel like camping in style there is Glacier Park KOA just outside the west entrance that is first class, they have a guy that comes in and cooks a great steak or fish dinner each night for not a bad price, Ice cream shop and all the usual amenities of that kind of park.
These are three of the most travelled parks in the west, so you can't expect any time without being surrounded by people, lots of tour buses, rental RV's etc. Most of the visitors seem to be on tight schedules and want to see everything in one day, The drive like crazy from one viewpoint to the next and all pack in like sardines to look at the geyser, mud hole or biggest ball of twine. They seem to forget to take the time to look at the beauty of the park itself as they travel, unless of course someone sees an animal of any kind, then a huge traffic jam happens instantly with cars abandoned with doors hanging open as they run with their cameras and selfie stick's ..lol If you want to have some fun, just pull over and point your camera toward a distant tree and see how fast people pull over to see what you might be looking at, they get so upset when you say a nice piece of pointy rock ..lol. We have always seen the best critters, photo opportunities etc. at completely random places that are not listed as points of interest. We have been to Yellowstone in every season many times with winter being our most favorite. If you have time to actually park and get out on a back country trail, all the crowds disappear and the inner beauty of the park comes alive.
If you can and if its open (usually around memorial day), the beartooth loop (Beartooth and Chief Joseph higways) is well worth the drive on the east side of Yellowstone park
If you use Gaia maps, don't forget to download maps in advance as there is not typically any cell service in the parks, also include the national parks layer which includes a digital version of the park map they hand you at the gate, much easy to see your location and what's coming up.
You really can't go wrong with any of these parks, just realize you can't see everything in one visit, which is Mother Natures way of saying y'all come back..
