Denise, I'm a regular Yellowstone visitor, and have a few points for you to consider:
It's a great idea to use a motorhome to travel TO Yellowstone, but I'm against motorhome travel THROUGH the park, simply because it's not set up for the size of a vehicle like that.
Driving in Yellowstone can be tricky due to the twisting, narrow roads, heavy traffic, and animals who may cross the road without warning. Sudden stops and traffic jams are common, often due to nothing more than Bambi standing alongside a roadway, and everyone stopping wherever they can to take a picture. Depending on the size of your motorhome and time of year you'd be traveling there, transiting the park can be pretty treacherous. There are few motorhome-sized turnouts, and some roads are in pretty poor shape. If you were driving in the park and you found a sudden reason to stop, (e.g. the aforementioned Bambi, or a geyser going off, or another "hurry up or we'll miss it" site), you may not find a suitable place to safely park a motorhome right then and there. Your safety has to be your primary concern.
There is only ONE campground in Yellowstone that has RV hookups. (Fishing Bridge Campground, by Yellowstone Lake.) Reservations there are a requirement, and need to be made about a year in advance, if not longer. Unless things have changed in the last year or two, all the other campgrounds inside the park are on a first-come basis, and they fill up fast. But even if they can be reserved ahead of time now, you may or may not find a campsite large enough to accommodate a motorhome. Showers in the park are few and far between, and if you used the one inside the motorhome, you'd need to find a dump station to empty the holding tanks.
Then there is the issue of uprooting the motorhome each day, packing everything up, and driving off to see the park. You'd constantly be packing or unpacking thngs, leveling the RV, and making sure things won't fly around the inside of the motorhome while you're driving. Add in the sheer number of miles driving through Yellowstone requires, and you'd quickly add to the allotted mileage your rental allows.
The thing about motorhomes is it's more about the JOURNEY, not the DESTINATION. (That's why they have an engine - they're designed to be moving.) If you want to see stuff between your home and Yellowstone, too, then I'd say go ahead and rent the RV, and deal with whatever follows. But if your primary concern is seeing the park itself, then here's an option to consider:
Stay in West Yellowstone. It's centrally located, easy to get to, and is closest to the major attractions in the Park. There are several RV parks there, grocery stores, shopping, a cool IMAX theater, and it's walking distance to just about everything in the town. Leave the RV in place, and rent a car there for traveling through the park for the few days you'll be there. (Unless you towed one with you behind the RV, which might be an option.) Regardless of how you do it, use a car to see Yellowstone. You'll be MUCH happier.
The West Entrance to the park is right there, and it's a 14 mile drive to Madison Junction. A right turn takes you toward Old Faithful and the southern park of the park. A left turn takes you north, toward the Norris Geyser Basin, Mammoth Hot Springs, and other scenic points. At the end of the day, you can drive back to West Yellowstone, and your RV is set up and waiting for you. Easy to deal with, and a MAJOR headache reducer.
Yellowstone is a must-see for everyone, and I can't get enough of the place. Your kids will love it, and so will you. But you need to think and plan ahead, or it can be a very dissatisfying experience.
Food for thought.
Dave
