As you head west from Fargo in eastern ND going towards West Yellowstone, try to visit Teddy Roosevelt NP in western ND; has some good CG's and a neat place to visit. Another stop that I recommend on your way west, is Fort Union on the MT/ND border, a very historic place near the mouth of the Yellowstone River; but don't remember if they have camping or not.
I second Eugene Mahoney park on I80 between Omaha and Lincoln. Great park -- you can reserve spots online through the Nebraska State Parks website. Only one issue -- if you will be coming through on a weekend night, it may be difficult to get a space. Popular place.
There is a KOA near Omaha on I80 at Gretna. I have heard some decent things about it, but have never been there.
If I were staying in Omaha and Fargo (as opposed to, say, within an hour's drive of them) I would choose these. Bluffs Run Casino across the river from downtown Omaha with easy access to Interstate. It's basically a paved parking lot with hookups but we don't spend much time in the RV while there. Short drive to Henry Doorly Zoo (one of my favorite places). We also like to gamble and take in a buffet at the surrounding casinos. Lindenwood CG wins my vote in Fargo. It's a pretty park, right on the Red River of the North, and right in Fargo.
Mahoney is the best option, but not if the OP is intent on cruising up I29.
Must be something happening up in ND to go that far north and then turn left.
Granted, I'm a bit biased being from Nebraska, but I'd rather take I80 across into WY or even better, wind your way up to the Black Hills on two lanes like HWY2
when I went to yellowstone I went up and across I-90 and dropped down into yellowstone from the north, which is not very far south of I-90.
and I would not go across Nebraska on I-80 as it is loaded with trucks. when My BIL went out that way often and he would take hwy 30 in Nebraska to avoid i-80. and if ones goal it to get to the park, piddling around on 2 lanes is not my cup of tea, especially since you can pop over a hill at any time and find a tractor towing a manure spreader in front of you.
If you're traveling up I-29, then I'd for sure use Bluffs Run Casino in Council Bluffs, IA or Lake Manawa State Park in Council Bluffs. That is the only option that is close to your route and is a good, no-frills option for an overnight stop.
If you want to stop on I-29 before you get to Omaha, there are a couple campgrounds in Nebraska City, NE. If you want to go slightly beyond Omaha on I-29 to stay, look at campground options in Onawa, IA or Craig, NE.