Depends upon what you see. On the south side,
Colter Bay is probably your best bet. On the West side (this seems to be most popular for West side) is
Grizzly RV Park in West Yellowstone, and on the north side is
Yellowstone RV Park in Gardiner, MT.
The Colter Bay is convenient to Old Faithful and a lot of the geo-thermal things to see which is mostly on the southern half. Grizzly is convenient to both the southern geo-thermal and the Northern Tier for animals, although it is further from both areas, it's probably the most "centrally" located. Yellowstone RV Park is closer to the animals. We're photographers and love the animals so we always stay at Yellowstone and have not looked any further. It is about a mile from the north entrance, but for us a straight shoot into the park, and we're on the northern part of the park early to see what we're there to see.
Yellowstone RV Park has nice showers (a little old, but clean), nice laundry, and if you call early (they open in April 1 for reservations), you can get a spot on the Yellowstone river, which is very nice (bald eagles and osprey hunt the river regularly for fish).
With your rig and length I wouldn't plan on staying inside the park. It will just be too tight and you would probably end up having to encroach on the neighbor's space. When you're there, trek over to Bridge Bay and take a look at the full-hookup sites and you'll be glad you're outside the park.
We'd rather be out traveling, but I still work so we're part-timers.
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