San diego to Yellowstone and then to Seattle

Have you been to Zion? Bryce? Tetons-Jackson area. These could be on your route.
We like to stay off the interstates. Think about highways like US 93 or US 95.
Yellowstone takes a while to see. If you don't have reservations think about them quick. It takes us a long day to day trip each of the loops.
West out of Yellowstone could be another experience. US 12 out of Missoula, MT area to Lewiston, ID for a view of Hells Canyon area and maybe trip up canyon in jet boat.
Washington has a diverse climate and terrain. Might think about northern route across WA on WA SR 20 or a little south US 2.
Have a good slow trip!

Consider spending some time in Oregon - maybe take the Coast road back to SD. Lots of outstanding State Parks on the beaches which are uncrowded and a major change from the S Calif beaches.

Nix 93&95, it'll be hot as h$&@ll at that time of the year in the desert. Stay on I15 at least thru StGeorge, UT.

Our first stop on that trip is usually the Casablanca RV park in Mesquite, Nv. The RV park is just a paved lot but they have a great pool complex, decent restaurants and a Starbucks.

Leave there early in the AM to get thru the Virgin River Gorge before the wind starts. Be prepared to get stuck in the Gorge. When there's trouble there is no way to detour traffic. I've been stuck there on a couple of occasions and it can turn into a furnace.

I'd continue on I15 thru Idaho to West Yellowstone, MT and enter the Park there. You might want to make day trips into Yellowstone. A nice CG to consider is Red Rock RV park in Island Park, ID on the shores of Henry's Lake.

When you've seen Yellowstone, drive south through the Park, out the south entrance and into Jackson and Grand Teton. You can head west from there toward Seattle when ready.

Enjoy the trip, I'm envious!

I agree with Oregon coast.Most of their state parks have partial or full hookups.I would think by now it will be hard to find availability for July, most of the reservations have already been done a while ago. We are also going to Yellowstone this year in July and we are staying in West Yellowstone at a private campground called Grizzly RV Park.Give them a call maybe you get lucky and find a spot or try within the Yellowstone park. I think they do not take reservations.In Oregon we liked Jessie M Honeyman state park. In California is hard to find campgrounds with full hookups. We love Humboldt Redwoods and Prairie Creek up Northern Ca. Check it out .Good luck and happy trails

Oregon state parks do take reservations, but may not for after Labor Day.

Agreed that you need to get reservations in/around Yellowstone PRONTO. Grizzly RV is a good choice for West Yellowstone; Red Rock RV and Henry's Lake (ID) SP are good backups. There are several other CGs in the area of West Yellowstone too - check rvparkreviews.com to learn about them. If you want to camp inside Yellowstone, there is only one CG with hookups, Fishing Bridge RV Park, and you will need to book that a month ago. (Seriously - though they may have a cancellation.) Most of the bigger no-hookup CGs inside the park are reservable as well, and should also be mostly booked long ago.

If you're interested in historical sites, a couple stops along the way could include the Oregon Trail Center in Baker City OR (a couple CGs in the area too - and best to take just the toad to the center; some wonderful & interesting exhibits on the trail in this museum), and the Fort Hall replica in Pocatello ID (again, a couple CGs in town, which is midway between SLC and West Yellowstone).

Keep in mind that Yellowstone is HUGE. It is a lot easier to see if you stay inside the Park. Otherwise the drives are very, very long. The best place to stay is Fishing Bridge which is centrally located and take a quarter of the park per day X 4 days. Then you get to see more and drive less. Fishing Bridge is the only campground with hookups so you'll need to get reservations yesterday. If you don't need hookups there are several nice campgrounds. Fishing bridge is basically a parking lot but it serves the purpose because you get up early and are off to sightsee all day and get back late so you won't be there much.

Another area I recommend is Northern New Mexico from Santa Fe and Chama to Southern Colorado in the Durango, Ouray, Silverton area. Beautiful country but you may not want to go that far East.

Anyway, just get out there and drive. There is so much beauty on your planned route. Don't think you can go wrong. Have a great time!

In the NW in August anywhere east of the Cascades is hot. That said, once you get to Washington coming from Yellowstone I highly recommend either WA20 or US2 heading west from Spokane and to cross the Cascades...both are very scenic...if you have the time. If not, take I-90 all way to Seattle...just another freeway like in S. Cal.

OP:
You need to decide what route types you would like to take. Those that desire to run the interstates miss a lot of the true beauty of the our country.
We like to run the main US highways avoiding the interstates as much as we can.
You should let us know what kind of locations you desire to see. Your time frame will give you ample time for side trips.

We appreciate all your great ideas.

I have been to Yellowstone many many times and have always stayed in the park. This time we are not. We are at the Red Rock RV park now, arrived today. Hosts are nice, campground is clean. There are many things to see on the way.

We start our trip 1 1/2 hours north of you but this is it. First stop is the Oasis RV resort in Vegas. It is on the south end of town. Very nice and a good place to plug in and cool off. Next is the KOA in Cedar City, Ut. Only a few hours past Vegas but if you get caught it construction or an accident you will be happy you have a spot. Be sure to get a Pizza from the guy in front of the Ace Hardware store. Adobe brick oven on a little trailer and it is the best, hands down.

Then we go to the KOA Brigham / Perry. It is good for a nights rest but not much more. And do NOT walk in the RV with your shoes on. The stickers on the ground are BAD and hurt like heck and get in the carpet.

From there it is 325 miles to the west gate of YS and another 50 to Fishing Bridge CG in the park. On the 20 there are wild animal park and other things to see. Just watch for the signs.

Beware of the construction on the 20 near Ashton. The road is narrow with a BAD drop on the edge. Another RV'er was going way too fast south bound and came in my lane. To avoid a head-on with the J.A. I had to hug the right ride of the road and my trailer went off the shoulder. I was lucky because on this curve it was only about a tapered edge and was able to save my rig. The rest of the construction has a 2' drop and there is no saving it when some JA is unsafe. Call the gal at the Red Rock and she will tell you how to go around it on a nice road.