Sorry, typo, 13 people - families of 5, 4, and 4. Still more than we can fit in two vehicles.
Honestly, if it's at all reasonable, touring the park in the motorhome is my first choice. It's the ultimate day trip vehicle, especially with young kids. I just didn't want to go into this blind and find out I can't take it outside of the road to and from the campground.
Regarding water tankage...our budget is such that for the difference in price between Grizzly RV or KOA and the sites at Bridge Bay, I'll gladly drive to the dump station as needed. I've also been told that staying in the park is preferable so that you don't need to deal with traffic in and out each day. Different priorities I guess.