First come first served state and national parks

x2 that April is too early for Yellowstone. Roads are literally closed to vehicles - other than snowcoaches - in April, the exception being between Gardiner and Cooke City MT, near the northern edge of the park. Plowing the many-feet-deep, packed-down snow takes weeks to finish. There are no hookups in Mammoth CG, so you will not be able to run your furnace and your water pipes in the camper will be at risk of freezing up. You may look into CGs in Gardiner, just outside the park; if they are open, you would have access to hookups and can drive into the northern part of the park from there. You will not be able to drive to the rest of the park (i.e. Old Faithful, the falls at Canyon, Lake area, etc.) at that time of year.

That said, you may want to do some research about specific parks in the other areas you plan to stay. Many state parks in the northern tier - as you know, being from Ohio - may be closed that early in the season as well, or will offer fewer services (i.e. no water, etc.) Flexibility will definitely be required as you travel. Those that are open, however, will probably have very few campers in them, and should be nice & quiet!

You will have no problem getting a site in Mammoth, because April is too early to be at Yellowstone. The first roads that open in Yellowstone do not open until April 17th. Many of the roads do not open until the first half of May, and some not until May 22nd. Be aware that is only the roads opening, not the related services. I doubt there will be any place to get water or dump tanks, the campgrounds with those facilities do not open until later. You can expect temps to be below freezing nightly in April. Personally, I would re-arrange my trip to do Yellowstone at a more benign time.

Get there early as in when the ranger first gets to the kiosk.
You may need to wait but if the campground can fill up you may as well get early pick before that long site you need goes to a tent camper that arrived first.
Worst case the ranger may put you on a list or give a later time to return.

Thanks very much everyone. I know April is early, but we're taking 2 months to go from Ohio south, west, north, and then back east. We're just trying to catch highlights and are expecting the cold. ;) Flexibility is the key, it seems.

Thanks again!

Mammoth Campground is open all year. It is at a lower elevation than most of the CG's in Yellowstone. Not as much snow, but could be cold in April.
You would most likely have to enter from the North Entrance, since that is the only entrance open all year. Most likely the road thru Lamar Valley will be the only road you can travel.

I think April is too EARLY.

We have been camp hosts in Grant for the last 4 seasons. April is a bit early and cold and if the campground is open you should have no problem getting a space. Grant is the last one to open and does not do so until late June due to the animal issues. During July & August most of the CG's in the park are full by 9-10 AM.

You might consider staying in West Yellowstone so you have full hook ups and can run your electric heaters.

Yellowstone in April might be tuff due to the snow.

I would be getting there late mornings. Many are packing up to leave by 10:00 at both places.
Plan B for Yellowstone would be Indian Creek.

Early afternoon. Have a plan B in mind though the time you mentioned should not be too bad