Thanks everyone for the info and insight. The reservation page does say the max length in all CG except Fishing Bridge is 40 feet including both TV and trailer. I guess I will just be honest with them when I make the reservation so they assign me an appropriate site.
Here is the info from Xterrra. Read the text and look at the picture which shows a rig much longer than 40 ft. Makes no sense to me.
Capacity for Grant Campground Small Tent and/or RV 40-foot Total or Less Site:
RV/Vehicle combination, 40' total length or less and
One small tent, 8'x8' or smaller
This is a single-width site. The total extended length of all vehicles and towed/towing equipment cannot exceed 40 feet
Grant Campground Quick Facts:
Located on West Thumb of Yellowstone Lake, adjacent to Grant Village.
Each site has a picnic table and fire grill.
Flush toilets are located nearby. Pay showers and coin laundry facilities are available at Camper Services, ¼- to 1-mile away.
General store, post office, gas station and Ranger Station are located nearby.
Nightly Park Service programs at amphitheater.
Check-in and check-out times are 11:00 a.m.
Other site types, including those for guests with disabilities, may be available by calling our reservations office at 1-307-344-7311.
More details about Grant Campground can be found here.